nano-malloc: Fix redefined compilation warning

When newlib is configured with --enable-newlib-reent-check-verify,
the assert macro is already defined in the nano-mallocr.c compile unit.

Contributed by STMicroelectronics

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@st.com>

newlib/libc/stdlib/nano-mallocr.c

@@ -40,6 +40,7 @@
 #if DEBUG
 #include <assert.h>
 #else
+#undef assert
 #define assert(x) ((void)0)
 #endif