glsl: use ralloc_str_append() rather than ralloc_asprintf_rewrite_tail()

glsl: use ralloc_str_append() rather than ralloc_asprintf_rewrite_tail()

The Deus Ex: Mankind Divided shaders go from spending ~20 seconds
in the GLSL IR compilers front-end down to ~18.5 seconds on a
Ryzen 1800X.

Tested by compiling once with shader-db then deleting the index file
from the shader cache and compiling again.

v2:
 - fix rebasing issue in v1

Reviewed-by: Thomas Helland <thomashelland90@gmail.com>