lwip: Fix C++ compilation error

Remove definition of DEBUG_PRINT as a C function when it is not
defined by user.

Previously DEBUG_PRINT was defined as printf when not defined by
user. This causes a C++ compilation error because of
"using std::printf" in stdio.h.
Redefining a symbol which is used in a "using" directive is
illegal.

Signed-off-by: Chris Swinchatt <christopher.swinchatt@arm.com>