package/libgpg-error: fix build without threads

Fix build without threads of libgpg-error in version >= 1.40

Fixes:
 - http://autobuild.buildroot.org/results/3344c96e5627a9327b0eabe0b27f34490bbabc0d



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>