msgfile: Fix warning on BSDs stdio

msgfile.c:116:16: warning: implicit declaration of function 'getline' is invalid in C99
      [-Wimplicit-function-declaration]
        while (getline(&line, &n, file) != -1) {
diff --git a/src/msgfile.c b/src/msgfile.c
index 1a99184..bf36bf3 100644
--- a/src/msgfile.c
+++ b/src/msgfile.c
@@ -21,6 +21,8 @@
  *
  */
 
+#define _WITH_GETLINE
+
 #include <osmocom/core/msgfile.h>
 #include <osmocom/core/talloc.h>