libtcgtpm: set -Wno-error=unused-variable in TPM CFLAGS

The next commit switches to TCG TPM, but there is a problem when the
`FILE_BACKED_NV` macro is set to `NO`. It produces the following compile
error because a variable is not used:

    Platform/src/NVMem.c: In function ‘_plat__NVDisable’:
    Platform/src/NVMem.c:185:9: error: unused variable ‘delete’ [-Werror=unused-variable]
      185 |     int delete = ((intptr_t)platParameter != 0)
          |         ^~~~~~
    cc1: all warnings being treated as errors
    make[2]: *** [Makefile:2674: Platform/src/libplatform_a-NVMem.o] Error 1

This commit can be reverted when the following PR will be merged:
TrustedComputingGroup/TPM#7

Signed-off-by: Stefano Garzarella <[email protected]>

libtcgtpm/Makefile

@@ -109,6 +109,11 @@ TCGTPM_CFLAGS += -static -nostdinc -fno-stack-protector -fPIE -mno-sse -mno-red-
 TCGTPM_CFLAGS += -DSIMULATION=NO -DFILE_BACKED_NV=NO
 TCGTPM_CFLAGS += -I$(LIBCRT_DIR)/include
 TCGTPM_CFLAGS += -I$(OPENSSL_DIR)/include
+# This is needed to workaround an issue with an unused variable when
+# FILE_BACKED_NV is set to NO. The real fix is posted here:
+# https://github.com/TrustedComputingGroup/TPM/pull/7
+# When that PR is merged, we can remove this line.
+TCGTPM_CFLAGS += -Wno-error=unused-variable
 
 # Configure the TPM 2.0 Reference Implementation and remove the pthread requirement.
 # In fact, pthread is required only in the TPM simulator, but we