drm trivial build fix

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

I found I needed this in the no options SYSCTL_INCLUDE_DESCR case, to
avoid -Werror.
OK to commit?

Cheers,

Patrick

Index: drm_sysctl.c
===================================================================
RCS file: /cvsroot/src/sys/external/bsd/drm2/drm/drm_sysctl.c,v
retrieving revision 1.3
diff -u -r1.3 drm_sysctl.c
--- drm_sysctl.c	12 Nov 2014 04:53:13 -0000	1.3
+++ drm_sysctl.c	24 Nov 2014 16:18:57 -0000
@@ -40,6 +40,7 @@
 
 #include <drm/drm_sysctl.h>
 
+#ifdef SYSCTL_INCLUDE_DESCR
 static const char *
 drm_sysctl_get_description(const struct linux_module_param_info *p,
     const struct drm_sysctl_def *def)
@@ -53,6 +54,7 @@
 	}
 	return NULL;
 }
+#endif
 
 #ifdef notyet
 static uint64_t