glsl: fix gcc 8 parenthesis warning

glsl: fix gcc 8 parenthesis warning

fixes warnings like this:
[184/1137] Compiling C++ object 'src/compiler/glsl/glsl@sta/lower_jumps.cpp.o'.
In file included from ../src/mesa/main/mtypes.h:48,
                 from ../src/compiler/glsl_types.h:149,
                 from ../src/compiler/glsl/lower_jumps.cpp:59:
../src/compiler/glsl/lower_jumps.cpp: In member function '{anonymous}::block_record {anonymous}::ir_lower_jumps_visitor::visit_block(exec_list*)':
../src/compiler/glsl/list.h:650:17: warning: unnecessary parentheses in declaration of 'node' [-Wparentheses]
    for (__type *(__inst) = (__type *)(__list)->head_sentinel.next; \
                 ^
../src/compiler/glsl/lower_jumps.cpp:510:7: note: in expansion of macro 'foreach_in_list'
       foreach_in_list(ir_instruction, node, list) {
       ^~~~~~~~~~~~~~~

Signed-off-by: Marc Dietrich <marvin24@gmx.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>