build fails on go >1.15.14

[MichaelMraka]

When building it fails with

rpmbuild.go:36:39: constant -2147483648 overflows RpmBuildFlag

This is because included rpmbuild.h defines
RPMBUILD_NOBUILD = (1 << 31) /*!< Don't execute or package. */
and go interpretes it as a signed int and then can't convert it.

Please merge a fix in #4 .