MBEDTLS_GCM_H macro

\file gcm.h This file contains GCM definitions and functions. The Galois/Counter Mode (GCM) for 128-bit block ciphers is defined in D. McGrew, J. Viega, The Galois/Counter Mode of Operation (GCM), Natl. Inst. Stand. Technol. For more information on GCM, see NIST SP 800-38D: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC.