libsel4: Turn seL4_MsgMaxLength into an enum.

This has no effect on the code, but makes this value visible as a named
constant in Isabelle when importing C sources. It avoids us having to reference
120 as a magic number in numerous proofs.

libsel4/include/sel4/constants.h

@@ -26,8 +26,9 @@ enum seL4_MsgLimits {
     seL4_MsgExtraCapBits = 2
 };
 
-
-#define seL4_MsgMaxLength 120
+enum {
+    seL4_MsgMaxLength = 120,
+};
 #define seL4_MsgMaxExtraCaps (BIT(seL4_MsgExtraCapBits)-1)
 
 #endif /* __API_CONSTANTS_H */