python,util: Fix magic number check in decode_inst_dep_trace.py (#560)

The decode_inst_dep_trace.py opens the trace file in read mode, and
subsequently reads the magic number from the trace file. Once the number
is read, it is compared against the string 'gem5' without decoding it
first. This causes the comparison to fail.
The fix addresses this by calling the decode() routine on the output of
the read() call. Please find the details in the associated issue #543

util/decode_inst_dep_trace.py

@@ -130,7 +130,7 @@ def main():
         exit(-1)
 
     # Read the magic number in 4-byte Little Endian
-    magic_number = proto_in.read(4)
+    magic_number = proto_in.read(4).decode()
 
     if magic_number != "gem5":
         print("Unrecognized file")