grep/pcre2: move definitions of pcre2_{malloc,free}

grep/pcre2: move definitions of pcre2_{malloc,free}

Move the definitions of the pcre2_{malloc,free} functions above the
compile_pcre2_pattern() function they're used in.

Before the preceding commit they used to be needed earlier, but now we
can move them to be adjacent to the other PCREv2 functions.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>