fix debian packages generation (invalid warning silencing)

During dh_shlibdeps, we silence some plugin-related warnings using
"grep -v". grep is the last command of the line, and returns 1 on match.
Therefore, the whole make step fails.

This patch merges the two grep commands, and ignores the grep return
value.

Change-Id: I1237162ab3c9937dbc340e5a2fce7ec779a19f39
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>

build-root/deb/debian/rules

@@ -32,8 +32,7 @@ override_dh_install:
 
 override_dh_shlibdeps:
 	@dh_shlibdeps 2>&1 \
-	  | grep -v "probably a plugin" \
-	  | grep -v "similar warnings have been skipped"
+	  | { grep -v -e "probably a plugin" -e "similar warnings have been skipped" || true; }
 
 override_dh_strip:
 	dh_strip --dbg-package=vpp-dbg