build: fix macOS --enable-modules build

build: fix macOS --enable-modules build

Apple's nm implementation includes empty lines in the output that are not
found in GNU binutils.  This confuses scripts/undefsym.py, though it did
not confuse the scripts/undefsym.sh script that it replaced.  To fix
this, ignore lines that do not have two fields.

Reported-by: Emmanuel Blot <eblot.ml@gmail.com>
Tested-by: Emmanuel Blot <eblot.ml@gmail.com>
Fixes: 604f3e4e90 ("meson: Convert undefsym.sh to undefsym.py", 2020-09-08)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>