Add cast to make room for naming pixel_format_t

Fixes a compiler warning/error about comparing two different enum
types so that the HAL_PIXEL_FORMAT_* enum in system/graphics.h can be
named.

Change-Id: I7acb5f67c66e42a4e556ff53c1e59cee40c7d585
diff --git a/media/libstagefright/ACodec.cpp b/media/libstagefright/ACodec.cpp
index 0b81b80..fad06b8 100644
--- a/media/libstagefright/ACodec.cpp
+++ b/media/libstagefright/ACodec.cpp
@@ -3840,7 +3840,7 @@
         fmt != OMX_COLOR_FormatYUV420PackedPlanar &&
         fmt != OMX_COLOR_FormatYUV420SemiPlanar &&
         fmt != OMX_COLOR_FormatYUV420PackedSemiPlanar &&
-        fmt != HAL_PIXEL_FORMAT_YV12) {
+        fmt != (OMX_COLOR_FORMATTYPE)HAL_PIXEL_FORMAT_YV12) {
         ALOGW("do not know color format 0x%x = %d", fmt, fmt);
         return false;
     }