Texture2DDecoderNative - GCC - Add missing header for memset

[KiruyaMomochi]

Texture2DDecoderNative fails to build with GNU toolchain due to memset undefined.
This pull request adds string.h to crn_depcomp.h so the compilation can success.

Note:

This is the minimum effort to make it compile with gcc when NDEBUG is set (i.e. in release mode).
However, compilation still fails when NDEBUG is not set or clang is used due to pointer cast problem, for example:

crunch/crn_decomp.h: In function ‘void* crnd::crnd_malloc(size_t, size_t*)’:
crunch/crn_decomp.h:2534:20: error: cast from ‘crnd::uint8*’ {aka ‘unsigned char*’} to ‘crnd::uint32’ {aka ‘unsigned int’} loses precision [-fpermissive]