fix clang compiler error

[ice6]

move variables decleard in header file to the C file to avoid clang compiler error.

below is the error:

duplicate symbol '_wkhtmltox_pdf_object_ce' in:
    .libs/wkhtmltox.o
    src/.libs/pdf.o
duplicate symbol '_wkhtmltox_pdf_ce' in:
    .libs/wkhtmltox.o
    src/.libs/pdf.o
duplicate symbol '_wkhtmltox_image_ce' in:
    .libs/wkhtmltox.o
    src/.libs/image.o
ld: 3 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)