libcamera: base: utils: Simplify operator<< for Duration

There is no real need for a function template. It is not defined in a
header file, so it has limited availability, and there exists only a
single instantion.

So convert it to use `std::ostream` directly, like most `operator<<`
in the code base.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/base/utils.h

@@ -424,9 +424,7 @@ scope_exit(EF) -> scope_exit<EF>;
 } /* namespace utils */
 
 #ifndef __DOXYGEN__
-template<class CharT, class Traits>
-std::basic_ostream<CharT, Traits> &operator<<(std::basic_ostream<CharT, Traits> &os,
-					      const utils::Duration &d);
+std::ostream &operator<<(std::ostream &os, const utils::Duration &d);
 #endif
 
 } /* namespace libcamera */