GeneratePPNInspiral.h includes itself

Per https://git.ligo.org/lscsoft/lalsuite/-/blob/master/lalinspiral/lib/GeneratePPNInspiral.h#L23, GeneratePPNInspiral.h appears to include itself. This issue prevents projects that use LAL from managing their dependencies with utilities such as include-what-you-use.

Merge request incoming! (edit: !2095 (merged))