Add missing C headers to timings.py

Signed-off-by: Clifford Wolf <clifford@clifford.at>

icetime/timings.py

diff --git a/icetime/timings.py b/icetime/timings.py
index 604dbcd..136f604 100644
--- a/icetime/timings.py
+++ b/icetime/timings.py
@@ -5,6 +5,8 @@
 
 print("// auto-generated by timings.py from ../icefuzz/timings_*.txt")
 print("#include <string>")
+print("#include <stdio.h>")
+print("#include <stdlib.h>")
 
 def timings_to_c(chip, f):
     print("")