mem-pool: drop trailing semicolon from macro definition

mem-pool: drop trailing semicolon from macro definition

Allow BLOCK_GROWTH_SIZE to be used like an integer literal by removing
the trailing semicolon from its definition.  Also wrap the expression in
parentheses, to allow it to be used with operators without leading to
unexpected results.  It doesn't matter for the current use site, but
make it follow standard macro rules anyway to avoid future surprises.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>