Update qmake files

As promised, here is me updating the qmake project files in order to
allow building the new version with qmake instead of cmake.

libs/qcustomplot-source/qcustomplot.pro

@@ -0,0 +1,10 @@
+TEMPLATE = lib
+
+CONFIG += staticlib
+CONFIG += debug_and_release
+
+HEADERS += \
+    qcustomplot.h
+
+SOURCES += \
+    qcustomplot.cpp