python3: fix build with SSL module

python3: fix build with SSL module

No idea how this creeped up. Probably OpenSSL been has updated recently.
Will send this patch upstream as well, but in the meantime we should fix
the Python3 build.

Build error seems to be:
```
<openwrt>/build_dir/target-i386_pentium4_musl/Python-3.7.0/Modules/_ssl.c:4000:5: error: implicit declaration of function 'DH_free'; did you mean 'lh_free'? [-Werror=implicit-function-declaration]
     DH_free(dh);
     ^~~~~~~
     lh_free
cc1: some warnings being treated as errors

Python build finished successfully!
The necessary bits to build these optional modules were not found:
_tkinter              _uuid                 nis
To find the necessary bits, look in setup.py in detect_modules() for the module's name.

The following modules found by detect_modules() in setup.py, have been
built by the Makefile instead, as configured by the Setup files:
_abc                  atexit                pwd
time                  zlib

Failed to build these modules:
_ssl

Makefile:618: recipe for target 'sharedmods' failed
```

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>