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 <baruch@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 <fontaine.fabrice@gmail.com>

Credit: 
Fabrice Fontaine

acinclude.m4

@@ -441,6 +441,7 @@ m4_case([$1],
 [mbedtls], [
   LIBSSH2_LIB_HAVE_LINKFLAGS([mbedcrypto], [], [#include <mbedtls/version.h>], [
     AC_DEFINE(LIBSSH2_MBEDTLS, 1, [Use $1])
+    LIBS="$LIBS -lmbedcrypto"
     found_crypto="$1"
     support_clear_memory=yes
   ])