libstagefright_foundation: fix build

Change-Id: If61766efa8834df175bd5758c760344364458d4a
diff --git a/media/libstagefright/foundation/AString.cpp b/media/libstagefright/foundation/AString.cpp
index 1befef4..bf474e1 100644
--- a/media/libstagefright/foundation/AString.cpp
+++ b/media/libstagefright/foundation/AString.cpp
@@ -358,7 +358,7 @@
 }
 
 status_t AString::writeToParcel(Parcel *parcel) const {
-    CHECK_LE(mSize, INT32_MAX);
+    CHECK_LE(mSize, (size_t)INT32_MAX);
     status_t err = parcel->writeInt32(mSize);
     if (err == OK) {
         err = parcel->write(mData, mSize);