streamripper: Fix Homebrew/NoFileutilsRmrf RuboCop offenses

[issyl0]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

---

• Also fix Sorbet error when ENV.cppflags is nil:

Error: An exception occurred within a child process:
    TypeError: Parameter 'newflags': Expected type String, got type NilClass
    Caller: /home/linuxbrew/.linuxbrew/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/s/streamripper.rb:31
    Definition: /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/extend/ENV/shared.rb:70 (SharedEnvExtension#append_to_cflags)

This failed to build still in the big PR of s* formulae.

[SMillerDev]

  lib/argv.c:126:7: error: call to undeclared library function 'strcpy' with type 'char *(char *, const char *)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        strcpy (copy[argc], argv[argc]);
        ^
  lib/argv.c:126:7: note: include the header <string.h> or explicitly provide a declaration for 'strcpy'
  1 error generated.