Allow using prebuilt flex and bison

build:soong:diff.txt

diff --git a/ui/build/paths/config.go b/ui/build/paths/config.go
index d17b9f77..6c124066 100644
--- a/ui/build/paths/config.go
+++ b/ui/build/paths/config.go
@@ -110,6 +110,8 @@ var Configuration = map[string]PathConfig{
        "xz":       Allowed,
        "zip":      Allowed,
        "zipinfo":  Allowed,
+       "flex":     Allowed,
+       "bison":    Allowed,
 
        // Host toolchain is removed. In-tree toolchain should be used instead.
        // GCC also can't find cc1 with this implementation.