Avoid another warning with gcc 4.8

While compiling I get this error:
```
In file included from /home/alessio/Desktop/buildrump.sh/src/lib/librumphijack/../librumpuser/rumpuser_port.h:55:0,
                 from /home/alessio/Desktop/buildrump.sh/src/lib/librumphijack/hijackdlsym.c:28:
/usr/include/features.h:330:4: error: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Werror=cpp]
 #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
    ^
cc1: all warnings being treated as errors
*** [hijackdlsym.pico] Error code 1
```
Fixed passing ``-Wno-cpp``.