Config: select PLATFORM_LINUX if using sendfile()

Man entry for sendfile:

  Not specified in POSIX.1-2001, or other standards.

  Other UNIX systems implement sendfile() with different  semantics  and
  prototypes. It should not be used in portable programs.

Select PLATFORM_LINUX if enabling FEATURE_USE_SENDFILE.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

Config.in

@@ -267,6 +267,7 @@ config PAM
 config FEATURE_USE_SENDFILE
 	bool "Use sendfile system call"
 	default y
+	select PLATFORM_LINUX
 	help
 	  When enabled, busybox will use the kernel sendfile() function
 	  instead of read/write loops to copy data between file descriptors