Fix build error: explicitly include <cstring> for std::strlen

[tsaito-fam]

When building with the latest versions of fmt and nameof, the following compiler error occurs:

error: no member named 'strlen' in namespace 'std'; did you mean simply 'strlen'?
[build]  3762 |             return read(str, std::strlen(str), read_flag);
[build]       |                              ^~~~~~~~~~~
[build]       |                              strlen
[build] /usr/include/string.h:439:15: note: 'strlen' declared here
[build]   439 | extern size_t strlen (const char *__s)

This appears to be caused by one of the transitive dependencies no longer including <cstring>, so std::strlen is no longer available in the current translation unit.

This change adds an explicit #include <cstring> to resolve the issue.

[yosh-matsuda]

@tsaito-fam Thank you for your contribution. This fix has been included in v0.7.0.