gbm: make 'devices' array static

It's only used in this one file as far as I can tell, and exporting a
symbol named 'devices' from a shared library is a recipe for trouble.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

src/gbm/main/gbm.c

@@ -43,7 +43,7 @@
 
 #define ARRAY_SIZE(a) (sizeof(a)/sizeof((a)[0]))
 
-struct gbm_device *devices[16];
+static struct gbm_device *devices[16];
 
 static int device_num = 0;