Fix warning during build regarding using operator == with two const char*'s

Steps to reproduce:

Build latest master (a3fcb9c4)

What happened?

During the build, this warning occurs:

src/extension/internal/pdfinput/svg-builder.cpp:884:100: warning: result of comparison against a string literal is unspecified (use an explicit string comparison function instead) [-Wstring-compare]
  884 |         bool prev_even_odd = prev_path->attribute("clip-rule") ? prev_path->attribute("clip-rule") == "evenodd" : false;
      |                                                                                                    ^  ~~~~~~~~~
1 warning generated.

What should have happened?

No warnings.

Inkscape Version and Operating System:

a3fcb9c4