Remove implicit cast from 'void *'

Commit e2809e3a10 added code to a header which assigns a pointer
to void to a pointer to unsigned char. This causes build errors for
extensions written in C++. Fix by adding an explicit cast.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CANWCAZaCq9AHBuhs%3DMx7Gg_0Af9oRU7iAqr0itJCtfmsWwVmnQ%40mail.gmail.com
Backpatch-through: 18

src/include/port/pg_crc32c.h

@@ -72,7 +72,7 @@ pg_comp_crc32c_dispatch(pg_crc32c crc, const void *data, size_t len)
 {
 	if (__builtin_constant_p(len) && len < 32)
 	{
-		const unsigned char *p = data;
+		const unsigned char *p = (const unsigned char *) data;
 
 		/*
 		 * For small constant inputs, inline the computation to avoid a