tools/pkgconf: add PKG_CONFIG_EXTRAARGS for additional arguments

This can be used my the OpenWrt build system to pass additional
arguments when required.

Signed-off-by: Andre Heider <a.heider@gmail.com>

tools/pkgconf/files/pkg-config

@@ -5,4 +5,5 @@ pkg-config.real \
 --keep-system-libs \
 --define-variable=prefix="${STAGING_PREFIX}" \
 --define-variable=exec_prefix="${STAGING_PREFIX}" \
---define-variable=bindir="${STAGING_PREFIX}/bin" "$@"
+--define-variable=bindir="${STAGING_PREFIX}/bin" \
+$PKG_CONFIG_EXTRAARGS "$@"