cmd/go: include Fortran files in build ID computation

Otherwise removing a .f file won't trigger a rebuild.
Noticed by inspection while working on the code.
I don't have a good way to write a test for this,
and I expect the code to change significantly in the next
release, but might as well get it right for Go 1.9.

Change-Id: I3f6f9f71b3a7d4f0be49a47419dac59899959e7c
Reviewed-on: https://go-review.googlesource.com/44855
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/load/pkg.go

@@ -1628,6 +1628,7 @@ func computeBuildID(p *Package) {
 		p.CgoFiles,
 		p.CFiles,
 		p.CXXFiles,
+		p.FFiles,
 		p.MFiles,
 		p.HFiles,
 		p.SFiles,