github: Install the diffutils package when building on msys2

The configure script of winpthreads tries to execute the "cmp"
command (pulled in via the libtool macros), visible as errors in
the build logs. (The issue doesn't seem to make any practical
difference though.)

.github/workflows/build.yml

@@ -161,6 +161,7 @@ jobs:
           install: >-
             git
             make
+            diffutils
           pacboy: >-
             toolchain:p
             cmake:p