Don't rely on transitively included headers

One must explicitly include what it need.
time.h for clock_gettime
float.h for FLT_MAX
string.h for memset

Bug: 37629934
Test: build

Change-Id: I1030a9c9ff3c2de4c5febabc3064d6976e9535cf

modules/hwcomposer/hwcomposer.cpp

@@ -17,6 +17,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <malloc.h>
+#include <string.h>
 
 #include <cutils/atomic.h>
 #include <log/log.h>