Fix: bracket expression matches a character at gawk

Fix: bracket expression matches a character at gawk

This is a trivial patch. However, It's must be modified by gawk.
Check it exactly as a plain character (e.g., space, alnum)
within the bracket expression at ./coreclr/generateversionscript.awk

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>