gnu: fasthenry: Fix build.

* gnu/packages/engineering.scm (fasthenry): Fix build by explicitly using
gcc-9 instead of the current gcc version.
[native-inputs]: Add gcc-9.

Change-Id: Ie2ef7fd5cd1e01ac4d273e6528e8e745f517b81f

gnu/packages/engineering.scm

@@ -774,6 +774,7 @@ multipole-accelerated algorithm.")
                                        "fasthenry-spUtils.patch"
                                        "fasthenry-spSolve.patch"
                                        "fasthenry-spFactor.patch"))))
+    (native-inputs (list gcc-9))
     (build-system gnu-build-system)
     (arguments
      `(#:make-flags '("CC=gcc" "RM=rm" "SHELL=sh" "all")