bug#43979: strerror return value

This is just a very minor issue.

libparted contains some calls to strerror where the result is assigned
to a char *. However, according to the strerror specification, the
string returned by strerror should never be changed, so it would be
better to use const char * instead.

This affects one line each in

libparted/arch/beos.c
libparted/arch/linux.c

Philipp