acinclude.m4: add mbedtls to LIBS (#371)
acinclude.m4: add mbedtls to LIBS (#371)

Notes: 
This is useful for static builds so that the Libs.private field in
libssh2.pc contains correct info for the benefit of pkg-config users.
Static link with libssh2 requires this information.

Signed-off-by: Baruch Siach <53ccad33e0625915318fb3bc832e5c1c413d1294@tkos.co.il>
[Retrieved from:
https://git.buildroot.net/buildroot/tree/package/libssh2/0002-acinclude.m4-add-mbedtls-to-LIBS.patch]
Signed-off-by: Fabrice Fontaine <7d88633730b8b85c1516e612893df72eae9cd341@gmail.com>

Credit: 
Fabrice Fontaine
libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2