Add include file needed when compiling with musl

diff --git a/include/asterisk/compat.h b/include/asterisk/compat.h
index 8547283..476dc2a 100644
--- a/include/asterisk/compat.h
+++ b/include/asterisk/compat.h
@@ -30,6 +30,7 @@
 #include <inttypes.h>
 #include <limits.h>
 #include <unistd.h>
+#include <pthread.h>
 
 #ifdef HAVE_STDDEF_H
 #include <stddef.h>