Missing include on musl - transitively included on glibc
https://bugs.gentoo.org/898174
https://bugs.gentoo.org/832766
--- a/src/wbt201.c
+++ b/src/wbt201.c
@@ -36,6 +36,7 @@
 #include <errno.h>
 #include <gdk/gdkkeysyms.h>
 #include <math.h>
+#include <locale.h>
 
 #define DEFDEVS "[g-rays2]\ndevices = auto;bluetooth;/dev/ttyUSB0\n"