chore: fix compilation pre-C++17 (#5)

`.data()` returns `const char*` rather than `char*` before C++17.
It's also not needed in this particular case.

src/certs.cc

@@ -53,7 +53,7 @@ void failOnError(OSStatus status, const char* error) {
           CFStringGetLength(str.get()),
 	        kCFStringEncodingUTF8) +
       1);
-    CFStringGetCString(str.get(), &msg.data()[offset], msg.size() - offset, kCFStringEncodingUTF8);
+    CFStringGetCString(str.get(), &msg[offset], msg.size() - offset, kCFStringEncodingUTF8);
     msg.resize(strlen(msg.data()));
     throw std::runtime_error(msg);
   }