Replace "depends on PLATFORM_LINUX" with "select PLATFORM_LINUX"

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/Config.src b/libbb/Config.src
index 18bdc51..a25af23 100644
--- a/libbb/Config.src
+++ b/libbb/Config.src
@@ -183,7 +183,7 @@
 config MONOTONIC_SYSCALL
 	bool "Use clock_gettime(CLOCK_MONOTONIC) syscall"
 	default n
-	depends on PLATFORM_LINUX
+	select PLATFORM_LINUX
 	help
 	  Use clock_gettime(CLOCK_MONOTONIC) syscall for measuring
 	  time intervals (time, ping, traceroute etc need this).