libgfortran: Fix compilation of gf_vsnprintf

libgfortran: Fix compilation of gf_vsnprintf

The fallback function (gf_vsnprintf) to provide a vsnprintf function
if the system library doesn't have one would not compile due to the
variable name for the string's destination buffer not being updated
after the refactor in 2018 in edaaef601d0d6d263fba87b42d6d04c99dd23dba.

This updates the internal logic of gf_vsnprintf to now use the str
variable defined in the function signature.

libgfortran/ChangeLog:

2024-04-04  Ian McInerney  <i.mcinerney17@imperial.ac.uk>

	* runtime/error.c (gf_vsnprintf): Fix compilation

Signed-off-by: Ian McInerney <i.mcinerney17@imperial.ac.uk>