Fix build error due to uninitialized variable warning

Fix build error due to uninitialized variable warning

As of commit a5fdf78a4402341a0ae23a5ca8a63825e2b5286c, building GDB with
a GCC 4.1 host compiler fails with:

gdb/cp-namespace.c: In function 'cp_lookup_symbol_via_imports':
gdb/cp-namespace.c:482: warning: 'sym.block' may be used uninitialized in this function

Apparently, more recent compilers are able to deduce that no actual
uninitialized use of sym.block takes place, but GCC 4.1 isn't yet
able to do that.

Fixed by adding an explicit initalization.

gdb/
	* cp-namespace.c (cp_lookup_symbol_via_imports): Fix uninitialized
	variable warning with some compilers.