util: fix asprintf() fallback

Eric Engestrom requested to merge eric/mesa:asprintf-fix into master

I double checked just after merging !1395 (merged), and that's when I noticed this:

warning C4047: 'function': 'const char *' differs in levels of indirection from 'char **'
warning C4024: 'util_asprintf': different types for formal and actual parameter 1

Turns out I completely messed up the function params and return type, and it didn't really bother MSVC which just went on, happily compiling nonsense 🙄

This time around AppVeyor isn't saying anything about asprintf, so it should be good.
Also, I actually checked the man this time and the function signature and behaviour matches the spec, so there's that too 🙃

Fixes: 9607d499 ("util: add asprintf() wrapper for MSVC")
/cc @anholt

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