util: Updated util/libdrm.h stubs with drmGetMagic()

Update adds in missing items that MacOS code needs to compile and run the DRI2 code.

New code is copied from the public repository: https://gitlab.freedesktop.org/mesa/drm/-/blob/main/xf86drm.h

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28130>

src/util/libdrm.h

@@ -32,6 +32,7 @@
 #else
 
 #include <errno.h>
+#include <stddef.h>
 #include <stdint.h>
 #include <sys/types.h>
 
@@ -45,6 +46,14 @@
 #define DRM_BUS_PLATFORM  2
 #define DRM_BUS_HOST1X    3
 
+typedef unsigned int drm_magic_t;
+
+static int
+drmGetMagic(int fd, drm_magic_t * magic)
+{
+  return -EINVAL;
+}
+
 typedef struct _drmPciDeviceInfo {
     uint16_t vendor_id;
     uint16_t device_id;