Add x11_headers dependency to all targets that use it

This fixes the build for me since I have libX11 installed to a custom
prefix, so the default include path does not include libX11. This fixes
lots of instances of the following error:
../../libglvnd/include/EGL/eglplatform.h:109:10: fatal error: 'X11/Xlib.h' file not found

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>

src/EGL/meson.build

@@ -25,6 +25,7 @@ libegl_dispatch_stubs = static_library(
   'egl_dispatch_stubs',
   ['egldispatchstubs.c', g_egldispatchstubs_c, g_egldispatchstubs_h],
   include_directories : inc_include,
+  dependencies : [dep_x11_headers],
   gnu_symbol_visibility : 'hidden',
 )
 

tests/dummy/meson.build

@@ -48,7 +48,7 @@ if get_option('egl')
       c_args : ['-DDUMMY_VENDOR_NAME="dummy@0@"'.format(v)],
       include_directories : [inc_include],
       link_with: [libpatchentrypoints],
-      dependencies : [idep_glvnd_pthread],
+      dependencies : [idep_glvnd_pthread, dep_x11_headers],
       version : '0',
     )
   endforeach

tests/meson.build

@@ -200,7 +200,7 @@ if get_option('egl')
         ['test@0@.c'.format(t[0]), 'egl_test_utils.c'],
         include_directories : [inc_include],
         link_with : [libEGL, t[1]],
-        dependencies : [t[2]],
+        dependencies : [t[2], dep_x11_headers],
       ),
       env : env_egl,
       suite : ['egl'],