Makefile: fix stray \ warnings with grep-3.8

We simply grep for "/usr". So no need for "-E" or "\/". Furthermore, in
the new grep versions this creates warnings.

As written in the grep-3.8 announcement:
  Regular expressions with stray backslashes now cause warnings, as
  their unspecified behavior can lead to unexpected results.
  For example, '\a' and 'a' are not always equivalent
  <https://bugs.gnu.org/39678>.

Fixes warnings in the form of:
  grep: warning: stray \ before /

Signed-off-by: Nick Hainke <[email protected]>

Makefile

@@ -14,7 +14,7 @@ $(if $(findstring $(space),$(TOPDIR)),$(error ERROR: The path to the OpenWrt dir
 
 world:
 
-DISTRO_PKG_CONFIG:=$(shell which -a pkg-config | grep -E '\/usr' -m 1)
+DISTRO_PKG_CONFIG:=$(shell which -a pkg-config | grep '/usr' -m 1)
 export PATH:=$(TOPDIR)/staging_dir/host/bin:$(PATH)
 
 ifneq ($(OPENWRT_BUILD),1)