fix compile error on 32bit systems

4c2766b (drm_mmap/drm_unmap) brought this error for every .c file that
was not #including config.h:

  In file included from private.h:4:0,
                   from abi16.c:29:
  ../libdrm.h: In function 'drm_munmap':
  ../libdrm.h:81:4: error: size of unnamed array is negative

Signed-off-by: Rob Clark <robdclark@gmail.com>
diff --git a/nouveau/abi16.c b/nouveau/abi16.c
index f34175e..ae13821 100644
--- a/nouveau/abi16.c
+++ b/nouveau/abi16.c
@@ -22,6 +22,10 @@
  * Authors: Ben Skeggs
  */
 
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
 #include <stdlib.h>
 #include <stdint.h>
 #include <stddef.h>