subdom: Fix -Werror=address failure in tmp_emulator

The opensuse-tumbleweed build jobs currently fail with:

  /builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.c: In function 'rsa_private':
  /builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.c:56:7: error: the comparison will always evaluate as 'true' for the address of 'p' will never be NULL [-Werror=address]
     56 |   if (!key->p || !key->q || !key->u) {
        |       ^
  In file included from /builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.c:17:
  /builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.h:28:12: note: 'p' declared here
     28 |   tpm_bn_t p;
        |            ^

This is because all tpm_bn_t's are 1-element arrays (of either a GMP or
OpenSSL BIGNUM flavour).

Adjust it to compile.  No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
(cherry picked from commit 46c5ef609b)

stubdom/Makefile

@@ -241,6 +241,7 @@ tpm_emulator-$(XEN_TARGET_ARCH): tpm_emulator-$(TPMEMU_VERSION).tar.gz
 	patch -d $@ -p1 < vtpm_extern.patch
 	patch -d $@ -p1 < vtpm-microsecond-duration.patch
 	patch -d $@ -p1 < vtpm-command-duration.patch
+	patch -d $@ -p1 < vtpm-tpm_bn_t-addr.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-tpm_bn_t-addr.patch

@@ -0,0 +1,18 @@
+All tpm_bn_t's are a 1-element array of one form or another, meaning the code
+below is tautological and triggers -Werror=address.
+
+diff -ru tpm_emulator-x86_64.orig/crypto/rsa.c tpm_emulator-x86_64/crypto/rsa.c
+--- tpm_emulator-x86_64.orig/crypto/rsa.c	2011-12-20 18:30:06.000000000 +0000
++++ tpm_emulator-x86_64/crypto/rsa.c	2023-08-03 20:44:17.379166284 +0100
+@@ -53,10 +53,7 @@
+   tpm_bn_init2(c, key->size);
+   tpm_bn_import(p, in_len, 1, in);
+ 
+-  if (!key->p || !key->q || !key->u) {
+-    /* c = p ^ d mod n */
+-    tpm_bn_powm(c, p, key->d, key->n);
+-  } else {
++  {
+     tpm_bn_init2(m1, key->size / 2);
+     tpm_bn_init2(m2, key->size / 2);
+     tpm_bn_init2(h, key->size);