meson: remove b_lundef option

Meson automatically adds "-undefined dynamic_lookup" to
shared_module build targets; b_lundef is only needed for
executables.  Therefore, we can remove this option.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson.build

 project('qemu', ['c'], meson_version: '>=0.55.0',
         default_options: ['warning_level=1', 'c_std=gnu99', 'cpp_std=gnu++11',
-                          'b_lundef=false','b_colorout=auto'],
+                          'b_colorout=auto'],
         version: run_command('head', meson.source_root() / 'VERSION').stdout().strip())
 
 not_found = dependency('', required: false)