[legal] Add a selection of FILE_LICENCE declarations

Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

src/include/stddef.h

@@ -1,6 +1,8 @@
 #ifndef STDDEF_H
 #define STDDEF_H
 
+FILE_LICENCE ( GPL2_ONLY );
+
 /* for size_t */
 #include <stdint.h>