Allow the host expr.

(Kernel builds require GNU unary + extension.)

Bug: https://issuetracker.google.com/131747477
Test: treehugger
Change-Id: I404db44d2ad0503d571d6f554a20445ab36aa7de
diff --git a/ui/build/paths/config.go b/ui/build/paths/config.go
index ed2d9ca..2c44fec 100644
--- a/ui/build/paths/config.go
+++ b/ui/build/paths/config.go
@@ -81,6 +81,7 @@
 	"dd":       Allowed,
 	"diff":     Allowed,
 	"egrep":    Allowed,
+	"expr":     Allowed,
 	"find":     Allowed,
 	"fuser":    Allowed,
 	"getopt":   Allowed,
@@ -134,7 +135,6 @@
 	"du":        LinuxOnlyPrebuilt,
 	"echo":      LinuxOnlyPrebuilt,
 	"env":       LinuxOnlyPrebuilt,
-	"expr":      LinuxOnlyPrebuilt,
 	"head":      LinuxOnlyPrebuilt,
 	"getconf":   LinuxOnlyPrebuilt,
 	"hostname":  LinuxOnlyPrebuilt,