Fix uninitialized warning

Fix uninitialized warning

Initialize variable to NULL before calling kmod_module_new_from_lookup().

libkmod/libkmod-module.c: In function 'kmod_module_new_from_lookup.part.4.constprop':
libkmod/libkmod-module.c:192:8: warning: 'depmod' may be used uninitialized in this function [-Wmaybe-uninitialized]
   list = kmod_list_prepend(list, depmod);
        ^
libkmod/libkmod-module.c:173:23: note: 'depmod' was declared here
   struct kmod_module *depmod;