pixman-private: include <float.h> only in C code

<float.h> is included unconditionally by pixman-private.h, which in
turn gets included by assembler files. Unfortunately, with certain C
libraries (like the musl C library), <float.h> cannot be included in
assembler files:

  CCLD     libpixman-arm-simd.la
/home/test/buildroot/output/host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/float.h: Assembler messages:
/home/test/buildroot/output/host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/float.h:8: Error: bad instruction `int __flt_rounds(void)'
/home/test/buildroot/output/host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/float.h: Assembler messages:
/home/test/buildroot/output/host/usr/arm-buildroot-linux-musleabihf/sysroot/usr/include/float.h:8: Error: bad instruction `int __flt_rounds(void)'

It turns out however that <float.h> is not needed by assembly files,
so we move its inclusion within the #ifndef __ASSEMBLER__ condition,
which solves the problem.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>