Link export plugin against libm (-lm)

The export plugin uses the pow() function from libm without linking against
it. It has worked so far because Geany itself has a link against libm, but
should that be removed in the future, this would fail to resolve symbols.

Signed-off-by: Chow Loong Jin <hyperair@debian.org>
Signed-off-by: Colomban Wendling <ban@herbesfolles.org>

plugins/Makefile.am

@@ -94,7 +94,7 @@ splitwindow_la_CFLAGS   = -DG_LOG_DOMAIN=\""SplitWindow"\"
 demoplugin_la_LIBADD    = $(GTK_LIBS)
 classbuilder_la_LIBADD  = $(GTK_LIBS)
 htmlchars_la_LIBADD     = $(GTK_LIBS)
-export_la_LIBADD        = $(GTK_LIBS)
+export_la_LIBADD        = $(GTK_LIBS) -lm
 saveactions_la_LIBADD   = $(GTK_LIBS)
 filebrowser_la_LIBADD   = $(GTK_LIBS)
 splitwindow_la_LIBADD   = $(GTK_LIBS)