package/oprofile: fix musl build

The build error has not yet been found by the autobuilders:

operf.cpp: In function ‘int __delete_old_previous_sample_data(const char*, const stat*, int, FTW*)’:
operf.cpp:862:10: error: ‘FTW_STOP’ was not declared in this scope
   return FTW_STOP;
          ^
operf.cpp:864:10: error: ‘FTW_CONTINUE’ was not declared in this scope
   return FTW_CONTINUE;
          ^
operf.cpp: In function ‘void convert_sample_data()’:
operf.cpp:899:41: error: ‘FTW_ACTIONRETVAL’ was not declared in this scope
                 int flags = FTW_DEPTH | FTW_ACTIONRETVAL;

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>