build: ignore declaration-after-statement warnings

| Makefile:37: recipe for target 'xtest' failed
| make: *** [xtest] Error 2
| make: *** Waiting for unfinished jobs....
| arith_taf.c: In function 'get_handle':
| arith_taf.c:56:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
|   int h = handle_get(&hdb, ptr);
|   ^~~
| arith_taf.c: In function 'ta_entry_arith_new_var':
| arith_taf.c:82:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
|   size_t len = TEE_BigIntSizeInU32(params[0].value.a);
|   ^~~~~~
| arith_taf.c: In function 'ta_entry_arith_new_fmm_var':
| arith_taf.c:129:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
|   size_t len = TEE_BigIntFMMSizeInU32(params[0].value.a);
|   ^~~~~~
| arith_taf.c: In function 'ta_entry_arith_free_handle':
| arith_taf.c:150:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
|   void *ptr = put_handle(params[0].value.a & ~HT_MASK);
|   ^~~~
| arith_taf.c: In function 'ta_entry_arith_from_octet_string':
| arith_taf.c:165:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
|   TEE_BigInt *big_int = lookup_handle(HT_BIGINT, params[0].value.a);
|   ^~~~~~~~~~
| arith_taf.c: In function 'ta_entry_arith_from_s32':
| arith_taf.c:181:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
|   TEE_BigInt *big_int = lookup_handle(HT_BIGINT, params[0].value.a);
|   ^~~~~~~~~~

etc.

Signed-off-by: André Draszik <andre.draszik@jci.com>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/host/xtest/Makefile b/host/xtest/Makefile
index 7e152ef..f226500 100644
--- a/host/xtest/Makefile
+++ b/host/xtest/Makefile
@@ -154,6 +154,7 @@
 	  -Wmissing-prototypes -Wnested-externs -Wpointer-arith \
 	  -Wshadow -Wstrict-prototypes -Wswitch-default \
 	  -Wwrite-strings \
+	  -Wno-declaration-after-statement \
 	  -Wno-missing-field-initializers -Wno-format-zero-length
 endif