Standardize prototypes of platform_memcpy/memset

As replacements of standard library functions, they should have the same
prototype, including return type.

While it doesn't usually matter when used directly, it does when the address
of the function is taken, as done with memset_func, used for implementing
mbedtls_platform_zeroize().
diff --git a/library/platform_util.c b/library/platform_util.c
index dad8376..349146d 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -79,7 +79,7 @@
 }
 #endif /* MBEDTLS_PLATFORM_ZEROIZE_ALT */
 
-void mbedtls_platform_memset( void *ptr, int value, size_t num )
+void *mbedtls_platform_memset( void *ptr, int value, size_t num )
 {
     /* Randomize start offset. */
     size_t start_offset = (size_t) mbedtls_platform_random_in_range( num );
@@ -93,10 +93,10 @@
     memset( (void *) ptr, data, start_offset );
 
     /* Perform the original memset */
-    memset( ptr, value, num );
+    return( memset( ptr, value, num ) );
 }
 
-void mbedtls_platform_memcpy( void *dst, const void *src, size_t num )
+void *mbedtls_platform_memcpy( void *dst, const void *src, size_t num )
 {
     /* Randomize start offset. */
     size_t start_offset = (size_t) mbedtls_platform_random_in_range( num );
@@ -107,7 +107,7 @@
     memcpy( (void *) ( (unsigned char *) dst + start_offset ),
             (void *) ( (unsigned char *) src + start_offset ),
             ( num - start_offset ) );
-    memcpy( (void *) dst, (void *) src, start_offset );
+    return( memcpy( (void *) dst, (void *) src, start_offset ) );
 }
 
 int mbedtls_platform_memcmp( const void *buf1, const void *buf2, size_t num )