Ports: Explicitly set PKG_CONFIG_PATH for curl

Unfortunately 06c42df broke building curl, which now discovers host
libraries via pkg-config. We already have a handful of ports that set
PKG_CONFIG_PATH this way so this seems like an acceptable solution to
fix the build for now, long term we should integrate this properly into
host_env and target_env instead of unconditionally stripping it (before)
or passing it through (now). Some ports legitimately do need host
pkg-config.

Ports/curl/package.sh

@@ -12,6 +12,8 @@ depends=(
     'zstd'
 )
 
+export PKG_CONFIG_PATH="${SERENITY_INSTALL_ROOT}/usr/local/lib/pkgconfig"
+
 configure() {
     mkdir -p curl-build
     cmake \