loci: constify most global variables

The compiler will now put them in rodata, which may be more efficient.
diff --git a/c_gen/c_match.py b/c_gen/c_match.py
index 1b5be1e..8c27bb5 100644
--- a/c_gen/c_match.py
+++ b/c_gen/c_match.py
@@ -338,7 +338,7 @@
     out.write(""")
 
 /* Indexed by version number */
-extern uint64_t of_match_incompat[4];
+extern const uint64_t of_match_incompat[4];
 """)
 
 
@@ -387,7 +387,7 @@
 }
 
 /* Indexed by version number */
-uint64_t of_match_incompat[4] = {
+const uint64_t of_match_incompat[4] = {
     -1,
     OF_MATCH_V1_INCOMPAT,
     OF_MATCH_V2_INCOMPAT,