grep: don't return an expression from pcre2_free()

grep: don't return an expression from pcre2_free()

Previously, the void pcre2_free() function in grep.c returned free().
While free() itself is void, afaict it's still an expression as per
section A.2.3, subsection 6.8.6 (jump-statement) in both C99 [1] and C11
[2]:

> return expression

Section 6.8.6.4 in C99 [1] and C11 [2] says that:

> A return statement with an expression shall not appear in a function
> whose return type is void.

The consequence of the old behavior was that developer builds with
pedantic errors enabled broke Git if PCRE2 was enabled and a
smart-enough compiler to detect these errors was used.  This commit
fixes pedantic builds of Git that enables --with-libpcre.

[1] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
[2] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf

Signed-off-by: Hans Jerry Illikainen <hji@dyntopia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>