Fix loading of extension functions on OS X.

For a non-GLX application, at least, we need to pull our extensions
using dlsym, not glXGetProcAddress().  The symptom was a segfault in
glXGetProcAddress().
diff --git a/src/dispatch_common.c b/src/dispatch_common.c
index 11498a7..2af95a3 100644
--- a/src/dispatch_common.c
+++ b/src/dispatch_common.c
@@ -405,6 +405,8 @@
 {
 #ifdef _WIN32
     return wglGetProcAddress(name);
+#elif defined(__APPLE__)
+    return epoxy_gl_dlsym(name);
 #else
     if (api.egl_handle) {
 #if PLATFORM_HAS_EGL