cargo: Ensure libssh2 from system is used

This forces the use of libssh2 by exporting the
LIBSSH2_SYS_USE_PKG_CONFIG environment variable.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>

recipes-devtools/cargo/cargo.inc

@@ -6,7 +6,8 @@ LIC_FILES_CHKSUM += " \
 file://LICENSE-THIRD-PARTY;md5=892ea68b169e69cfe75097fc38a15b56 \
 "
 
-# Used in libgit2-sys's build.rs, needed for pkg-config to be used
+# Needed for pkg-config to be used
 export LIBGIT2_SYS_USE_PKG_CONFIG = "1"
+export LIBSSH2_SYS_USE_PKG_CONFIG = "1"
 
 BBCLASSEXTEND = "native"