stubdom: fix vtpm compilation on GCC-7

GCC-7 have -Wimplicit-fallthrough enabled with -Wextra. Add appropriate
comment which both mute the warning and improve readibility.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

stubdom/Makefile

@@ -228,6 +228,7 @@ tpm_emulator-$(XEN_TARGET_ARCH): tpm_emulator-$(TPMEMU_VERSION).tar.gz
 	patch -d $@ -p1 < vtpm-deepquote.patch
 	patch -d $@ -p1 < vtpm-deepquote-anyloc.patch
 	patch -d $@ -p1 < vtpm-cmake-Wextra.patch
+	patch -d $@ -p1 < vtpm-implicit-fallthrough.patch
 	mkdir $@/build
 	cd $@/build; CC=${CC} $(CMAKE) .. -DCMAKE_C_FLAGS:STRING="-std=c99 -DTPM_NO_EXTERN $(TARGET_CPPFLAGS) $(TARGET_CFLAGS) -Wno-declaration-after-statement"
 	touch $@

stubdom/vtpm-implicit-fallthrough.patch

@@ -0,0 +1,10 @@
+--- tpm_emulator-x86_64/tpm/tpm_cmd_handler.c.orig	2017-04-27 13:37:14.408000000 +0200
++++ tpm_emulator-x86_64/tpm/tpm_cmd_handler.c	2017-04-27 13:39:53.585000000 +0200
+@@ -3397,6 +3397,7 @@
+                   sizeof(rsp->auth2->nonceOdd.nonce));
+       tpm_hmac_update(&hmac, (BYTE*)&rsp->auth2->continueAuthSession, 1);
+       tpm_hmac_final(&hmac, rsp->auth2->auth);
++      /* fall-thru */
+     case TPM_TAG_RSP_AUTH1_COMMAND:
+       tpm_hmac_init(&hmac, rsp->auth1->secret, sizeof(rsp->auth1->secret));
+       tpm_hmac_update(&hmac, rsp->auth1->digest, sizeof(rsp->auth1->digest));