BUILD: tools: avoid a build warning on gcc-4.8 in resolve_sym_name()

BUILD: tools: avoid a build warning on gcc-4.8 in resolve_sym_name()

A build warning is emitted with gcc-4.8 in tools.c since commit
e920d73f59 ("MINOR: tools: improve symbol resolution without dl_addr")
because the compiler doesn't see that <size> is necessarily initialized.
Let's just preset it.

(cherry picked from commit ed75148ca0479c7f87e1c149f0b5f0c8dd50eb3c)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 19b041e30ad9be9f00588d83c7d36a405f1e9cca)
Signed-off-by: Willy Tarreau <w@1wt.eu>