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
diff --git a/include/hardware/thermal.h b/include/hardware/thermal.h
index b5b6e1d..5db6ee0 100644
--- a/include/hardware/thermal.h
+++ b/include/hardware/thermal.h
@@ -19,6 +19,7 @@
 
 #include <stdbool.h>
 #include <stdint.h>
+#include <float.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>