libssh2: fix pkg-config info for mbedtls backend

libssh2: fix pkg-config info for mbedtls backend

The libssh2.pc file did not contain the needed info for static link with
libssh2. Add a patch fixing that.

Fixes (qemu):
http://autobuild.buildroot.net/results/634/6346b25be2844f9ef722e52040ac1b43d9c38899/

Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit ebbf8746243ee4fa6b51a5a6afa8b14459b4178f)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>