libsecret: specify where libgcrypt lives

When libgcrypt support is enabled the configure script will try to
find libgcrypt-config in the PATH. If the host distribution has it then
brokeness ensues. Fixes:
http://autobuild.buildroot.net/results/eca/eca10b8360354e7e51406f7ac942d343987bde5e/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>