pinentry: Some toolchains need -pthread in the linker command line

Some toolchains, like Sourcery Codebench 2013.05, need -pthread to be
added to the linker command line when linking pinentry-qt.

Fixes:
   http://autobuild.buildroot.net/results/6be/6be109ccedec603a67cebdb31b55865dcce0e128/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Markos Chandras <Markos.Chandras@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>