util/disk_cache: Fix disk_cache_get_function_timestamp with disabled cache.

radv always needs it, so just check the header instead. Also
do not declare the function if the variable is not set, so we
get a nice compile error instead of failing to open a device
at runtime.

Fixes: b87ef9e606 "util: fix MSVC build issue in disk_cache.h"
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

meson.build

@@ -960,7 +960,7 @@ elif cc.has_header_symbol('sys/mkdev.h', 'major')
   pre_args += '-DMAJOR_IN_MKDEV'
 endif
 
-foreach h : ['xlocale.h', 'sys/sysctl.h', 'linux/futex.h', 'endian.h']
+foreach h : ['xlocale.h', 'sys/sysctl.h', 'linux/futex.h', 'endian.h', 'dlfcn.h']
   if cc.compiles('#include <@0@>'.format(h), name : '@0@'.format(h))
     pre_args += '-DHAVE_@0@'.format(h.to_upper().underscorify())
   endif