[libgfortran, patch] Silence a warning in fallback round() implementation

[FX]

PR 47970 is about a warning in libgfortran/intrinsics/c99_functions.c for 
platforms without round() and ceill(), because round() is used in roundl() 
before it's prototype is given. Moving the round() prototype and implementation 
earlier in the file fixes it.

Patch tested by manually editing my config.h on x86_64-linux to simulate the 
conditions. OK to commit to trunk?
FX

round.diff
Description: Binary data

round.ChangeLog
Description: Binary data