directsoundsrc: fix build error for 64bit Windows build by MinGW

Casting to UINT from HMIXER generates the following warning with
64bit Windows target MinGW:

    gstdirectsoundsrc.c: In function 'gst_directsound_src_mixer_find':
    gstdirectsoundsrc.c:733:30: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
         mmres = mixerGetDevCaps ((UINT) dsoundsrc->mixer,
                                  ^
    cc1: all warnings being treated as errors

We can use portable GPOINTER_TO_UINT() macro for this propose.

https://bugzilla.gnome.org/show_bug.cgi?id=754756
diff --git a/sys/directsound/gstdirectsoundsrc.c b/sys/directsound/gstdirectsoundsrc.c
index 0bd77e8..18f49f8 100644
--- a/sys/directsound/gstdirectsoundsrc.c
+++ b/sys/directsound/gstdirectsoundsrc.c
@@ -730,7 +730,7 @@
     if (mmres != MMSYSERR_NOERROR)
       continue;
 
-    mmres = mixerGetDevCaps ((UINT) dsoundsrc->mixer,
+    mmres = mixerGetDevCaps (GPOINTER_TO_UINT (dsoundsrc->mixer),
         mixer_caps, sizeof (MIXERCAPS));
 
     if (mmres != MMSYSERR_NOERROR) {