select.c: Add missing #include lines for string.h and sys/select.h

This is needed on Nuttx.
diff --git a/src/select.c b/src/select.c
index b29e5c5..b0e8b0c 100644
--- a/src/select.c
+++ b/src/select.c
@@ -21,6 +21,8 @@
 
 #include <fcntl.h>
 #include <stdio.h>
+#include <string.h>
+#include <sys/select.h>
 
 #include <osmocom/core/select.h>
 #include <osmocom/core/linuxlist.h>