package/libssh2: fix static linking scenarios involving mbedtls

package/libssh2: fix static linking scenarios involving mbedtls

curl can be statically linked with mbedtls, in this case build will
fail on:
kex.c:(.text+0x1be0): undefined reference to `mbedtls_mpi_read_binary'

This is due to the fact that CURL_LIBRARIES does not contain mbedtls
library:
CURL_LIBRARIES:INTERNAL=curl;cares;ssh2;ssh2;z;ssl;crypto;z;z;crypto;z;z;ssl;z;z;crypto;z

even if libcurl.pc is correct:
Libs.private: -lcares -lssh2 -L/home/fabrice/buildroot/output/host/bin/../arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib -lssh2 /home/fabrice/buildroot/output/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libmbedcrypto.a /home/fabrice/buildroot/output/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libmbedcrypto.a -L/home/fabrice/buildroot/output/host/bin/../arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib -L/home/fabrice/buildroot/output/host/bin/../arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib -lz -lssl -lcrypto -lssl -lz -lz -lcrypto -lz -lz

This full library path is added by patch
0002-acinclude.m4-add-mbedtls-to-LIBS.patch on libssh2 so update it to
replace $LIBMBDEDCRYPTO by $LTLIBMBEDCRYPTO as suggested by Thomas
during review of https://patchwork.ozlabs.org/patch/989339

Fixes:
 - http://autobuild.buildroot.org/results/dc7810d5d5c62658837cdd2faae6fe3390f968a2

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>