plug-ins: link file-fits with libm.

When linking with clang/lld, I had the following linking error:

> ld.lld: error: undefined symbol: pow

As a side change, also adding math.h header include, even though it
didn't seem to break compilation. Just to be thorough.

plug-ins/file-fits/fits.c

@@ -36,6 +36,7 @@
 
 #include "config.h"
 
+#include <math.h>
 #include <string.h>
 #include <errno.h>
 

plug-ins/file-fits/meson.build

@@ -24,6 +24,7 @@ if cfitsio_dep.found()
                           dependencies: [
                            libgimpui_dep,
                            cfitsio_dep,
+                           math,
                           ],
                           install: true,
                           install_dir: gimpplugindir / 'plug-ins' / plugin_name)