fix(r-rgsl): build

This fixes non-fatal build errors mentioning pkg-config not found, and a fatal undefined reference linker error.

packages/r-rgsl/package.py

@@ -21,3 +21,4 @@ class RRgsl(RPackage):
     # GNU Scientific Library: https://www.gnu.org/software/gsl
     # spack: https://spack.readthedocs.io/en/latest/package_list.html#gsl
     depends_on("gsl")
+    depends_on("pkgconfig")