spirv: initialize is_vertex_input

spirv: initialize is_vertex_input

Fixes warning:

../../src/compiler/spirv/vtn_variables.c: In function ‘var_decoration_cb’:
../../src/compiler/spirv/vtn_variables.c:1400:12: warning: ‘is_vertex_input’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       bool is_vertex_input;
            ^~~~~~~~~~~~~~~

The code used to set is_vertex_input in all possible codepaths, but
after 23edc5b1ef3 "spirv: translate default-block uniforms" the
compiler isn't sure all codepaths will initialize the variable.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>