picocom: fix build for x86 with musl libc

picocom: fix build for x86 with musl libc

Since version 3.1 picocom enabled by default custom baud rate support
for x86 targets. This requires the c_ispeed and c_opseed field in struct
termios that musl libc does not provide. Add an upstream patch that
restricts custom baud rates to C libraries that provide the needed
termios fields.

Fixes:
http://autobuild.buildroot.net/results/28e/28ec584484dedfd6ef473dfd9dd24481e27ce2b3/
http://autobuild.buildroot.net/results/912/912493a8f99416524a5897634ae62604436e9b51/
http://autobuild.buildroot.net/results/65d/65d0f449822f296dfe6b9be0e03a5a855f700a9e/

Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>