sdk: fix host menu config targets using ncurses

This applies 965f341aa9 ("build: fix host menu config targets using
ncurses") to the SDK top Makefile.

If there is a pkg-config in the staging dir, it will try to use it
instead of the host system's pkg-config; then it will fail to find the
ncurses package.  Linux's default will be used, which fails in some
cases, such as recent Gentoo systems.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>
[fixed From: to match SoB]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/sdk/files/Makefile

@@ -14,6 +14,7 @@ export TOPDIR LC_ALL LANG SDK
 
 world:
 
+DISTRO_PKG_CONFIG:=$(shell which -a pkg-config | grep -E '\/usr' | head -n 1)
 export PATH:=$(TOPDIR)/staging_dir/host/bin:$(PATH)
 
 ifneq ($(OPENWRT_BUILD),1)