util/disk_cache: fix size subtraction on 32bit
util/disk_cache: fix size subtraction on 32bit

Negating size_t on 32bit produces a 32bit result. This was effectively
adding values close to UINT_MAX to the cache size (the files are usually
small) instead of intended subtraction.
Fixes 'make check' disk_cache failures on 32bit.

Signed-off-by: Grazvydas Ignotas <eac34e836c3bc120048c86fead6dd0257a872a9e@gmail.com>
Reviewed-by: Timothy Arceri <c48760dafc316d20cf5b0817150c999e82cd892f@itsqueeze.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler