/usr/include/poppler/splash/ is in libpoppler-private-dev 0.48.0-2+deb9u2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/poppler/splash/Splash.h
/usr/include/poppler/splash/SplashBitmap.h
/usr/include/poppler/splash/SplashClip.h
/usr/include/poppler/splash/SplashErrorCodes.h
/usr/include/poppler/splash/SplashFTFont.h
/usr/include/poppler/splash/SplashFTFontEngine.h
/usr/include/poppler/splash/SplashFTFontFile.h
/usr/include/poppler/splash/SplashFont.h
/usr/include/poppler/splash/SplashFontEngine.h
/usr/include/poppler/splash/SplashFontFile.h
/usr/include/poppler/splash/SplashFontFileID.h
/usr/include/poppler/splash/SplashGlyphBitmap.h
/usr/include/poppler/splash/SplashMath.h
/usr/include/poppler/splash/SplashPath.h
/usr/include/poppler/splash/SplashPattern.h
/usr/include/poppler/splash/SplashScreen.h
/usr/include/poppler/splash/SplashState.h
/usr/include/poppler/splash/SplashT1Font.h
/usr/include/poppler/splash/SplashT1FontEngine.h
/usr/include/poppler/splash/SplashT1FontFile.h
/usr/include/poppler/splash/SplashTypes.h
/usr/include/poppler/splash/SplashXPath.h
/usr/include/poppler/splash/SplashXPathScanner.h