modem-manager: fix build with musl

modem-manager: fix build with musl

modem-manager version 1.8.0 introduced use of canonicalize_file_name(),
which is a GNU extension that musl does not implement. Add a patch
switching to POSIX standard realpath().

Fixes:
http://autobuild.buildroot.net/results/9f6/9f6859f1854d94d3caba7b5c24d9ca6a14622042/

Cc: Aleksander Morgado <aleksander@aleksander.es>
Cc: Carlos Santos <casantos@datacom.com.br>
Cc: Petr Vorel <petr.vorel@gmail.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>