mongoose: bump to version 5.4

Avoid compilation of the server via the upstream Makefile which
unconditionally uses -lssl

The -lssl flag is appended to MONGOOSE_CFLAGS whenever
BR2_PACKAGE_OPENSSL=y gets set.

Signed-off-by: Davide Viti <zinosat@tiscali.it>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>