Try to fix compile error on windows

https://github.com/ruby/ruby/runs/1041040167?check_suite_focus=true#step:11:177
```
compiling ../src/re.c
re.c
../src/re.c(317): error C2057: expected constant expression
../src/re.c(317): error C2466: cannot allocate an array of constant size 0
../src/re.c(467): error C2057: expected constant expression
../src/re.c(467): error C2466: cannot allocate an array of constant size 0
../src/re.c(467): error C2133: 'opts': unknown size
../src/re.c(559): error C2057: expected constant expression
../src/re.c(559): error C2466: cannot allocate an array of constant size 0
../src/re.c(559): error C2133: 'optbuf': unknown size
../src/re.c(673): error C2057: expected constant expression
../src/re.c(673): error C2466: cannot allocate an array of constant size 0
../src/re.c(673): error C2133: 'opts': unknown size
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.27.29110\bin\HostX64\x64\cl.EXE"' : return code '0x2'
Stop.
```

1 files changed, 1 insertions, 1 deletions

diff --git a/re.c b/re.c
index d0dbfaaa29..badd321782 100644
--- a/re.c
+++ b/re.c
@@ -311,7 +311,7 @@ char_to_option(int c)
     return val;
 }
 
-static const int OPTBUF_SIZE = 4;
+enum { OPTBUF_SIZE = 4 };
 
 static char *
 option_to_str(char str[OPTBUF_SIZE], int options)