CLEANUP: proxy: fix duplicate declaration of cli_find_frontend in proxy.h

CLEANUP: proxy: fix duplicate declaration of cli_find_frontend in proxy.h

The function cli_find_frontend was declared twice identically at lines 98-99
of include/haproxy/proxy.h. The second declaration should have been for
cli_find_backend, which is defined in src/proxy.c and used in several places
but was missing from the header's exported symbols.

This is a simple copy-paste mistake where line 99 duplicated line 98 verbatim
instead of declaring cli_find_backend.