nautilus-global-preferences: use 'extern' properly

Variables in header files should have 'extern' on them and be defined
from a single .c file, otherwise you get a copy of the variable every
time you #include the header.

-fcommon masks this problem, but building with -fno-common fails because
of it.

https://bugzilla.gnome.org/show_bug.cgi?id=704117