Change Linux specific check to linux-gnu

When swift-ci ran the tests, it looks like `%target-os` was "linux-gnu”, which wasn’t listed in the Testing Swift document (ony listing `macosx`, `darwin`, `linux`, `freebsd`, `windows-cygnus`, and `windows-gnu`)

Since I can’t (at least don’t know how to) run the linux tests myself, I’ll try and update the check-prefix to see if that passes.

Author: David Rönnqvist

test/ClangImporter/non-modular-include.swift

@@ -3,7 +3,7 @@
 
 // CHECK: {{.+}}/non-modular/Foo.framework/Headers/Foo.h:1:9: error: include of non-modular header inside framework module 'Foo'
 // CHECK-macosx: error: could not build Objective-C module 'Foo'
-// CHECK-linux: error: could not build C module 'Foo'
+// CHECK-linux-gnu: error: could not build C module 'Foo'
 // CHECK-NOT: error
 
 import Foo