Fix a leak in XCreateFontSet

a simple snippet like XFreeFontSet(d, XCreateFontSet(d, ...)) will generate lots of memory leaks,
as evidenced by the following valgrind output:
==983== HEAP SUMMARY:
==983==     in use at exit: 39,409 bytes in 341 blocks
==983==   total heap usage: 4,795 allocs, 4,454 frees, 489,086 bytes allocated
==983==
==983== 1,688 (136 direct, 1,552 indirect) bytes in 1 blocks are definitely lost in loss record
40 of 46
==983==    at 0x4C2B042: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==983==    by 0x56D5A93: add_codeset.clone.9 (in /usr/lib64/libX11.so.6.3.0)
==983==    by 0x56D5FE0: load_generic (in /usr/lib64/libX11.so.6.3.0)
==983==    by 0x56D7612: initialize (in /usr/lib64/libX11.so.6.3.0)
==983==    by 0x56D7E75: _XlcCreateLC (in /usr/lib64/libX11.so.6.3.0)
==983==    by 0x56F9A5F: _XlcUtf8Loader (in /usr/lib64/libX11.so.6.3.0)
==983==    by 0x56DF815: _XOpenLC (in /usr/lib64/libX11.so.6.3.0)
==983==    by 0x56B255A: XOpenOM (in /usr/lib64/libX11.so.6.3.0)
==983==    by 0x56A665A: XCreateFontSet (in /usr/lib64/libX11.so.6.3.0)
==983==    by 0x4FCA80: conky::x11_output::create_gc() (x11.cc:746)
==983==    by 0x4FC3B4: conky::x11_output::use_own_window() (x11.cc:602)
==983==    by 0x4FAD42: conky::priv::own_window_setting::set(bool const&, bool) (x11.cc:92)
==983==
==983== LEAK SUMMARY:
==983==    definitely lost: 136 bytes in 1 blocks
==983==    indirectly lost: 1,552 bytes in 34 blocks
==983==      possibly lost: 0 bytes in 0 blocks
==983==    still reachable: 37,721 bytes in 306 blocks
==983==         suppressed: 0 bytes in 0 blocks

This patch makes the leak dissappear (Well, at least the "definitely lost part". The "still
reachable" thingy remains). After some analysis, I've discovered that the XLCd structure is
destroyed improperly. The "constructor" is in lcGeneric.c, but the structure is destroyed using
code from lcPublic.c. I've found that changing the destructor call to _XlcDestroyLC executes the
correct code path, and I'm pretty sure this is correct (the object was constructed using
_XlcCreateLC, it make sense to destroy it using its conterpart).

So far I haven't observed any strange behaviour on my system caused by this change (although, I'm
not sure, how many programs actually use this function).

Signed-off-by: Pavel Labath <pavelo@centrum.sk>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>