glsl2: Don't declare a variable called sig that shadows the other one

glsl2: Don't declare a variable called sig that shadows the other one

Accidentally having a variable called 'sig' within an if-statement
cause the higher scope 'sig' to always be NULL.  As a result a new
function signature was created for a function definition even when one
already existed from a prototype declaration.

Fixes piglit test case glsl-function-prototype (bugzilla #29520).