build: fix warnings from grep

Fixes build warnings when using newer versions of grep.

Signed-off-by: Chris Osgood <[email protected]>
Tested-by: Georgi Valkov <[email protected]>

target/sdk/files/Makefile

@@ -14,7 +14,7 @@ export TOPDIR LC_ALL LANG SDK
 
 world:
 
-DISTRO_PKG_CONFIG:=$(shell $(TOPDIR)/scripts/command_all.sh pkg-config | grep -E '\/usr' -m 1)
+DISTRO_PKG_CONFIG:=$(shell $(TOPDIR)/scripts/command_all.sh pkg-config | grep '/usr' -m 1)
 export PATH:=$(TOPDIR)/staging_dir/host/bin:$(PATH)
 
 ifneq ($(OPENWRT_BUILD),1)