perf tools: Pass EXTRA_CFLAGS through to libbpf build again

perf tools: Pass EXTRA_CFLAGS through to libbpf build again

When perf was statically linked to libbpf, any definition of
EXTRA_CFLAGS passed to Makefile.perf propagated to the sub-make of
libbpf.  Since commit 9dabf4003423 ("perf python: Switch module to
linking libraries from building source"), EXTRA_CFLAGS is overridden
to "-fPIC" for the sub-make.

Change to include any user-provided EXTRA_CFLAGS before the "-fPIC"
option.

Fixes: 9dabf4003423 ("perf python: Switch module to linking libraries ...")
Signed-off-by: Ben Hutchings <benh@debian.org>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name perf-tools-pass-extra_cflags-through-to-libbpf-build-again.patch