[gdb/build] Fix -Werror=bool-compare warning in update_static_array_size

With current trunk I run into:
...
src/gdb/gdbtypes.c: In function 'bool update_static_array_size(type*)':
src/gdb/gdbtypes.c:1250:64: error: comparison of constant '0' with boolean \
  expression is always true [-Werror=bool-compare]
    && get_array_bounds (element_type, &low_bound, &high_bound) >= 0
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
...

Fix this by dropping the compare.

gdb/ChangeLog:

2020-11-18  Tom de Vries  <tdevries@suse.de>

	* gdbtypes.c (update_static_array_size): Fix -Werror=bool-compare
	warning.