Pack structure OEMCrypto_PST_Report

This change is copied from the widevine CL:
https://widevine-internal-review.googlesource.com/#/c/10163/

Because the OEMCrypto_PST_Report is sent as a signed block to the
server, it needs to be a fixed, platform independent, size.  This CL
adds the packed attribute to the structure, which reduces its size
from 56 bytes to 47 bytes.

Change-Id: I2bae058b7eb0ac54ba9fad355f3d85ddc2cd4a58

libwvdrmengine/oemcrypto/include/OEMCryptoCENC.h

@@ -215,7 +215,7 @@ typedef struct {
   uint8_t clock_security_level;
   uint8_t pst_length;
   uint8_t pst[];
-} OEMCrypto_PST_Report;
+} __attribute__((packed)) OEMCrypto_PST_Report;
 
 /*
  *  OEMCrypto_Clock_Security_Level.