Fix compilation error on musl

_XOPEN_SOURCE is needed for the strings.h ffs function on the musl libc implementation

types/data_device/wlr_data_offer.c

@@ -1,3 +1,4 @@
+#define _XOPEN_SOURCE 700
 #include <assert.h>
 #include <stdlib.h>
 #include <strings.h>