Fix undefined pthread_sigmask warning on OpenBSD

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5530 c046a42c-6fe2-441c-8c8c-71466251a162
diff --git a/audio/sdlaudio.c b/audio/sdlaudio.c
index b6cfad1..aa226f0 100644
--- a/audio/sdlaudio.c
+++ b/audio/sdlaudio.c
@@ -29,6 +29,8 @@
 #ifndef _WIN32
 #ifdef __sun__
 #define _POSIX_PTHREAD_SEMANTICS 1
+#elif defined(__OpenBSD__)
+#include <pthread.h>
 #endif
 #include <signal.h>
 #endif