patch 9.0.1788: C4090 warnings in strings.c

patch 9.0.1788: C4090 warnings in strings.c

Problem:  C4090 warnings in strings.c
Solution: Add type casts

closes: #12917

MSVC shows the following warnings:
```
strings.c(2436): warning C4090: 'function': different 'const' qualifiers
strings.c(2774): warning C4090: 'function': different 'const' qualifiers
strings.c(3865): warning C4090: 'function': different 'const' qualifiers
```

So add type casts to suppress them.

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Ken .Takata <kentkt@csc.jp>