This fixes pthread_mutex_t being undefined on musl libc.

https://github.com/brailcom/speechd/commit/17e4038948f01e389cbd7fedbe3f0eb54ad3292d
https://bugs.gentoo.org/833347
--- a/src/modules/module_main.h
+++ b/src/modules/module_main.h
@@ -31,6 +31,7 @@
 
 #include <speechd_types.h>
 #include <spd_audio.h>
+#include <pthread.h>
 
 #ifdef __cplusplus
 extern "C" {