Fix minimal-printf stack overflow

Author: evedon

platform/source/minimal-printf/mbed_printf_implementation.c

@@ -220,10 +220,10 @@ static void mbed_minimal_formatted_string_hexadecimal(char *buffer, size_t lengt
             unsigned int nibble_one = (output >> 4);
             unsigned int nibble_two = (output & 0x0F);
 
-            const char int2hex_lower[16] = { '0', '1', '2', '3', '4', '5', '6', '7',
+            static const char int2hex_lower[16] = { '0', '1', '2', '3', '4', '5', '6', '7',
                                              '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'
                                            };
-            const char int2hex_upper[16] = { '0', '1', '2', '3', '4', '5', '6', '7',
+            static const char int2hex_upper[16] = { '0', '1', '2', '3', '4', '5', '6', '7',
                                              '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'
                                            };
             const char *int2hex = upper ? int2hex_upper : int2hex_lower;