ac/gpu_info: Fix missing prototype mingw error

Fixes: 246051ebc6 ("ac/gpu_info: print 32bpp modifiers")

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

Closes #8858

(cherry picked from commit 795a36325a)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32261>

src/amd/common/ac_gpu_info.c

@@ -346,7 +346,7 @@ static intptr_t readlink(const char *path, char *buf, size_t bufsiz)
 {
    return -1;
 }
-extern char *
+static char *
 drmGetFormatModifierName(uint64_t modifier)
 {
    return NULL;