Fix warning-avoidance initialization in xcoffread.c

Fix warning-avoidance initialization in xcoffread.c

With the registry rewrite series, on Fedora 34, I started seeing this
error in xcoffread.c:

../../binutils-gdb/gdb/xcoffread.c: In function ‘void read_xcoff_symtab(objfile*, legacy_psymtab*)’:
../../binutils-gdb/gdb/xcoffread.c:948:25: error: ‘main_aux’ is used uninitialized [-Werror=uninitialized]
  948 |   union internal_auxent fcn_aux_saved = main_aux;
      |                         ^~~~~~~~~~~~~
../../binutils-gdb/gdb/xcoffread.c:933:25: note: ‘main_aux’ declared here
  933 |   union internal_auxent main_aux;
      |                         ^~~~~~~~

I don't know why this error started suddenly... that seems weird,
because it's not obviously related to the changes I made.

Looking into it, it seems this line was intended to avoid a similar
warning -- but since 'main_aux' is uninitialized at the point where it
is used, this fix was incomplete.

This patch avoids the warning by initializing using "{}".  I'm
checking this in.