Add define for CONST.

Newer interfaces take CONST parameters, so define CONST to minimize
differences between our headers and the standards docs.

Differential Revision: https://reviews.freebsd.org/D19971

stand/efi/include/efidef.h

@@ -63,6 +63,7 @@ typedef VOID            *EFI_EVENT;
     #define IN
     #define OUT
     #define OPTIONAL
+    #define CONST const
 #endif