Fix failure on make -j >1 as build_ext needs the result of cython

[felixonmars]

The failure was:

Running Cython
Building extension
==============
==================

cython --version

python setup.py build_ext --inplace --force
Cython version 0.23.4
cython --cplus --fast-fail --annotate plyvel/_plyvel.pyx
running build_ext
building 'plyvel._plyvel' extension
creating build
creating build/temp.linux-x86_64-3.5
creating build/temp.linux-x86_64-3.5/plyvel
gcc -pthread -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -D_FORTIFY_SOURCE=2 -fPIC -I/usr/include/python3.5m -c plyvel/_plyvel.cpp -o build/temp.linux-x86_64-3.5/plyvel/_plyvel.o -Wall -g
gcc: error: plyvel/_plyvel.cpp: No such file or directory
gcc: fatal error: no input files
compilation terminated.
error: command 'gcc' failed with exit status 1
Makefile:15: recipe for target 'ext' failed
make: *** [ext] Error 1
make: *** Waiting for unfinished jobs....

Fixed after applying the dependency.

[landscape-bot]

Repository health increased by 4% when pulling 25a0030 on felixonmars:master into e1818c1 on wbolster:master.

• No new problems were introduced.
• 6 problems were fixed (including 2 errors and 4 code smells).

[wbolster]

Yep, thanks.