Declare libinjection_sqli_check_fingerprint() from libinjection_sqli.h

because it's located there

to avoid this. i think it could at least partially solve #5
```
localhost:/opt/pylibinjection # python setup.py build
running build
running build_ext
cythoning src/pylibinjection.pyx to src/pylibinjection.c
warning: src/pylibinjection.pxd:103:44: Function signature does not match previous declaration
building 'pylibinjection' extension
gcc -pthread -fno-strict-aliasing -g -O2 -DNDEBUG -fmessage-length=0 -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector -funwind-tables -fasynchronous-unwind-tables -g -fPIC -I/opt/libinjection/c -I/usr/include/python2.7 -c src/pylibinjection.c -o build/temp.linux-x86_64-2.7/src/pylibinjection.o
src/pylibinjection.c: In function ‘__pyx_pf_14pylibinjection_detect_sqli’:
src/pylibinjection.c:770:3: error: ‘sfilter’ undeclared (first use in this function)
src/pylibinjection.c:770:3: note: each undeclared identifier is reported only once for each function it appears in
src/pylibinjection.c:770:12: error: ‘__pyx_v_sfp’ undeclared (first use in this function)
src/pylibinjection.c:791:28: error: expected expression before ‘)’ token
src/pylibinjection.c:827:3: warning: implicit declaration of function ‘libinjection_sqli_init’
src/pylibinjection.c:836:3: warning: implicit declaration of function ‘libinjection_is_sqli’
src/pylibinjection.c:848:3: warning: implicit declaration of function ‘libinjection_sqli_check_fingerprint’
error: command 'gcc' failed with exit status 1
```