tests/glheaders: use #if for platform selection

GST_GL_HAVE_* are always defined to 0 or 1 so an #ifdef will always
succeed which is not the intention here.
diff --git a/tests/check/libs/gstglheaders.c b/tests/check/libs/gstglheaders.c
index 1e4470d..287956e 100644
--- a/tests/check/libs/gstglheaders.c
+++ b/tests/check/libs/gstglheaders.c
@@ -64,7 +64,7 @@
 #include <gst/gl/egl/gstglmemoryegl.h>
 #endif
 
-#ifdef GST_GL_HAVE_PLATFORM_GLX
+#if GST_GL_HAVE_PLATFORM_GLX
 #include <gst/gl/x11/gstgldisplay_x11.h>
 #include <gst/gl/x11/gstglwindow_x11.h>
 #endif