Squelch warning about an integer overflow

We cannot rely on long integers to have more than 32 bits.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -475,7 +475,7 @@ extern int git_munmap(void *start, size_t length);
 #endif
 
 #define DEFAULT_PACKED_GIT_LIMIT \
-	((1024L * 1024L) * (sizeof(void*) >= 8 ? 8192 : 256))
+	((1024L * 1024L) * (size_t)(sizeof(void*) >= 8 ? 8192 : 256))
 
 #ifdef NO_PREAD
 #define pread git_pread