contrib/bc: fix build with GCC

Building with GCC failed with the following error message:

 error: to be safe all intermediate pointers in cast from 'char **'
 to 'const char **' must be 'const' qualified [-Werror=cast-qual]

This was caused by main() being declared with "char *argv[]" as the
3rd parameter, but argv later being passed cast to "const char**":

113 |     if (BC_IS_BC) s = bc_main(argc, (const char**) argv);
    |                                      ^

This is fixed by declaring the 3rd parameter of main() as "const
char *argv[]".

Reported by:	CI
MFC after:	3 days

contrib/bc/src/main.c

@@ -54,7 +54,7 @@
 #include <dc.h>
 
 int
-main(int argc, char* argv[])
+main(int argc, const char* argv[])
 {
 	BcStatus s;
 	char* name;