Fixed mingw build for portaudio

git-svn-id: https://svn.pjsip.org/repos/pjproject/branches/split-3rd-party@1213 74dad513-b988-da41-8d7b-12977e46ad98
diff --git a/third_party/build/portaudio/src/portaudio.h b/third_party/build/portaudio/src/portaudio.h
new file mode 100644
index 0000000..7ba2bcf
--- /dev/null
+++ b/third_party/build/portaudio/src/portaudio.h
@@ -0,0 +1 @@
+#include "../../../portaudio/include/portaudio.h"