libcamera: utils: Use internal secure_getenv() implementation

The secure_getenv() call is not provided by all C libraries. Support
this feature by implementing our own version.

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

src/libcamera/include/utils.h

@@ -24,6 +24,8 @@ std::unique_ptr<T> make_unique(Args&&... args)
 	return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
 }
 
+char *secure_getenv(const char *name);
+
 } /* namespace utils */
 
 } /* namespace libcamera */