Makefile.am: explicitly link with libdrm

Otherwise, a failure happens with gold linker:

../src/.libs/libvirglrenderer.so: error: undefined reference to 'drmPrimeHandleToFD'

https://errors.yoctoproject.org/Errors/Details/222046/

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/Makefile.am

@@ -1,5 +1,6 @@
 SUBDIRS := gallium/auxiliary
 AM_LDFLAGS = -lm \
+	$(LIBDRM_LIBS) \
 	$(GBM_LIBS) \
 	$(EPOXY_LIBS) \
 	$(X11_LIBS) \