util: Makefile.sources: Add disk_cache_os.{c,h}

The functions in disk_cache_os are referenced by disk_cache but these
files are not included in builds based on Makefile.sources such as
Android, resulting in linker errors.

Fixes: 4339ecde35 ("disk_cache: move cache dir generation into OS specific helper file")
Signed-off-by: Marijn Suijten <marijns95@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6702>

src/util/Makefile.sources

@@ -9,7 +9,7 @@ MESA_UTIL_FILES := \
 	blob.h \
 	build_id.c \
 	build_id.h \
-        compiler.h \
+	compiler.h \
 	crc32.c \
 	crc32.h \
 	dag.c \
@@ -18,6 +18,8 @@ MESA_UTIL_FILES := \
 	debug.h \
 	disk_cache.c \
 	disk_cache.h \
+	disk_cache_os.c \
+	disk_cache_os.h \
 	double.c \
 	double.h \
 	enum_operators.h \