Explicitly ignore unused variables (fixes #6158)

A macro in py/bc.h declares five variables that are used to hold data
temporarily, without their values being used after the assignments. This
causes "unused-but-set-variable" warnings in clang 13. We mark these
variables as explicitly ignored to avoid this new warning.

py/bc.h

@@ -124,6 +124,11 @@
             D |= (z & 0x1) << n;                                    \
         }                                                           \
         S += 1;                                                     \
+        (void)E;                                                    \
+        (void)F;                                                    \
+        (void)A;                                                    \
+        (void)K;                                                    \
+        (void)D;                                                    \
     } while (0)
 
 #define MP_BC_PRELUDE_SIG_DECODE(ip) \