py-gsl: add missing build dependency

Closes: https://trac.macports.org/ticket/72478

Author: reneeotten

python/py-gsl/Portfile

@@ -29,6 +29,7 @@ python.pep517_backend   meson
 if {${name} ne ${subport}} {
     depends_build-append \
                         port:swig \
+                        port:swig-python \
                         path:bin/pkg-config:pkgconfig
 
     depends_lib-append  port:gsl \