st/dri: build the drm backend when libdrm is present

Prevent build issues on systems lacking libdrm.
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>

src/gallium/state_trackers/dri/Makefile.am

@@ -20,4 +20,8 @@
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 # DEALINGS IN THE SOFTWARE.
 
-SUBDIRS = drm sw
+SUBDIRS = sw
+
+if HAVE_LIBDRM
+SUBDIRS += drm
+endif