valgrind: fix build for glibc 2.19

Fix build for glibc 2.19-based toolchains, fixes:
http://autobuild.buildroot.net/results/be4/be4c033e2088c324f93fc70230f05d761108cfba/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>