This fixes some compiler warnings with compiler flag
-Wmissing-variable-declarations (tested with clang):

    aarch64_be-linux-user-gdbstub-xml.c:564:19: warning: no previous extern declaration for non-static variable 'xml_builtin' [-Wmissing-variable-declarations]
    aarch64-linux-user-gdbstub-xml.c:564:19: warning: no previous extern declaration for non-static variable 'xml_builtin' [-Wmissing-variable-declarations]
    aarch64-softmmu-gdbstub-xml.c:1763:19: warning: no previous extern declaration for non-static variable 'xml_builtin' [-Wmissing-variable-declarations]

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>