Makefile: Set compiler switch for USE_NSEC

The comments indicated that setting a Makefile variable USE_NSEC would
enable the code for sub-second [cm]times.  However, the Makefile
variable was never turned into a compiler switch so the code was never
enabled.  This patch allows USE_NSEC to be noticed by the compiler.

Signed-off-by: Brian Gernhardt <benji@silverinsanity.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -932,6 +932,9 @@ endif
 ifdef NO_ST_BLOCKS_IN_STRUCT_STAT
 	BASIC_CFLAGS += -DNO_ST_BLOCKS_IN_STRUCT_STAT
 endif
+ifdef USE_NSEC
+	BASIC_CFLAGS += -DUSE_NSEC
+endif
 ifdef USE_ST_TIMESPEC
 	BASIC_CFLAGS += -DUSE_ST_TIMESPEC
 endif