tigervnc: Specify include directory for drm_fourcc.h

[Fix the error below.][1]:

```
Making all in dri3
make[1]: Entering directory '/dev/shm/bee-root/tigervnc/tigervnc-1.9.0-0/build/unix/xserver/dri3'
  CC       dri3.lo
  CC       dri3_screen.lo
  CC       dri3_request.lo
dri3.c:29:10: fatal error: drm_fourcc.h: No such file or directory
 #include <drm_fourcc.h>
          ^~~~~~~~~~~~~~
compilation terminated.
dri3_screen.c:32:10: fatal error: drm_fourcc.h: No such file or directory
 #include <drm_fourcc.h>
          ^~~~~~~~~~~~~~
compilation terminated.
dri3_request.c:33:10: fatal error: drm_fourcc.h: No such file or directory
 #include <drm_fourcc.h>
          ^~~~~~~~~~~~~~
compilation terminated.
make[1]: *** [Makefile:663: dri3.lo] Error 1
make[1]: *** Waiting for unfinished jobs....
make[1]: *** [Makefile:663: dri3_screen.lo] Error 1
make[1]: *** [Makefile:663: dri3_request.lo] Error 1
make[1]: Leaving directory '/dev/shm/bee-root/tigervnc/tigervnc-1.9.0-0/build/unix/xserver/dri3'
```

[1]: https://github.com/TigerVNC/tigervnc/issues/740

tigervnc.be0

@@ -74,6 +74,7 @@ mee_configure() {
         start_cmd cd unix/xserver
         start_cmd patch -p1 < $S/unix/xserver$XORG_SERVER_NNN.patch
         start_cmd autoreconf -fiv
+        export CFLAGS="$CFLAGS -I/usr/include/drm"
         start_cmd ./configure $DEFCONFIG \
             --with-pic --without-dtrace --disable-static --disable-dri \
             --disable-xinerama --disable-xvfb --disable-xnest --disable-xorg \