glsl: add missing initialization of the location path field

glsl: add missing initialization of the location path field

This was apparently missed in 67b32190f3c95, which added support
for ARB_shading_language_include to #line, including the 'path'
field for the location.

Fixes crashes in CTS with all drivers as they attempt to access
an uninitialized path string during parsing.

Fixes: 67b32190f3c95 ("glsl: add ARB_shading_language_include support to #line")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2132
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Jose Maria Casanova <jmcasanova@igalia.com>