Fix wrong prototype of getNativeScaleFactor() in systemScale.h

The fix for "8149115: [hidpi] Linux: display-wise scaling factor should probably be taken into account" changed the prototype for getNativeScaleFactor() in systemScale.h from:

int getNativeScaleFactor();

to

double getNativeScaleFactor();

But forgot to add the "char*" parameter to the signature which is required by the implementation in systemScale.c (and the various call sites):

double getNativeScaleFactor(char *output_name) {

This leads to the following waring/error:

/OpenJDK/jdk9-hs-comp-s390x/jdk/src/java.desktop/unix/native/libawt_xawt/awt/awt_GraphicsEnv.c: In function ‘Ja
va_sun_awt_X11GraphicsDevice_getNativeScaleFactor’:
/OpenJDK/jdk9-hs-comp-s390x/jdk/src/java.desktop/unix/native/libawt_xawt/awt/awt_GraphicsEnv.c:2185:5: error: c
all to function ‘getNativeScaleFactor’ without a real prototype [-Werror=unprototyped-calls]
     double scale = getNativeScaleFactor(name);
     ^
In file included from /OpenJDK/jdk9-hs-comp-s390x/jdk/src/java.desktop/unix/native/libawt_xawt/awt/awt_Graphics
Env.c:46:0:
/OpenJDK/jdk9-hs-comp-s390x/jdk/src/java.desktop/unix/native/common/awt/systemscale/systemScale.h:29:8: note: ‘
getNativeScaleFactor’ was declared here
 double getNativeScaleFactor();
        ^
cc1: all warnings being treated as errors

If we fix that we will get:

/usr/work/d046063/OpenJDK/jdk9-hs-comp-s390x/jdk/src/java.desktop/unix/native/libsplashscreen/splashscreen_sys.c: In function ‘S
plashGetScaledImageName’:
/usr/work/d046063/OpenJDK/jdk9-hs-comp-s390x/jdk/src/java.desktop/unix/native/libsplashscreen/splashscreen_sys.c:809:5: error: t
oo few arguments to function ‘getNativeScaleFactor’
     *scaleFactor = getNativeScaleFactor();
     ^

I observed these warnings/errors with 4.8.5 and it is a miracle for me why newer versions of GCC (apparently 4.9.2 which seems to be Oracles officially supported compiler according to https://wiki.openjdk.java.net/display/Build/Supported+Build+Platforms) or gcc 5.4.0 whiohc I've tried on Ubuntu 16.04 don't detect this problem?