benchmark: Fix incompatibility with C89 compilers

Initializing arrays using non-constant expressions is not permitted in
C89, and was causing errors when compiling with Metrowerks CodeWarrior
(for classic MacOS) in C89 mode. Clang also produces a warning when
compiling with '-Wc99-extensions':

test/benchmark.c:670:42: warning: initializer for aggregate is not a compile-time constant [-Wc99-extensions]
        const unsigned char *dhm_P[] = { dhm_P_2048, dhm_P_3072 };
                                         ^~~~~~~~~~
test/benchmark.c:674:42: warning: initializer for aggregate is not a compile-time constant [-Wc99-extensions]
        const unsigned char *dhm_G[] = { dhm_G_2048, dhm_G_3072 };
                                         ^~~~~~~~~~

Declaring the arrays as 'static' makes them constant expressions.

fixes #1353
diff --git a/programs/test/benchmark.c b/programs/test/benchmark.c
index 2864caf..1945b30 100644
--- a/programs/test/benchmark.c
+++ b/programs/test/benchmark.c
@@ -658,13 +658,13 @@
     if( todo.dhm )
     {
         int dhm_sizes[] = { 2048, 3072 };
-        const unsigned char dhm_P_2048[] =
+        static const unsigned char dhm_P_2048[] =
             MBEDTLS_DHM_RFC3526_MODP_2048_P_BIN;
-        const unsigned char dhm_P_3072[] =
+        static const unsigned char dhm_P_3072[] =
             MBEDTLS_DHM_RFC3526_MODP_3072_P_BIN;
-        const unsigned char dhm_G_2048[] =
+        static const unsigned char dhm_G_2048[] =
             MBEDTLS_DHM_RFC3526_MODP_2048_G_BIN;
-        const unsigned char dhm_G_3072[] =
+        static const unsigned char dhm_G_3072[] =
             MBEDTLS_DHM_RFC3526_MODP_3072_G_BIN;
 
         const unsigned char *dhm_P[] = { dhm_P_2048, dhm_P_3072 };