patch 8.2.4370: MS-Windows: libsodium.dll not included with the installer

Problem:    MS-Windows: libsodium.dll not included with the installer.
Solution:   Add the file to the installer if it exists. (Christian Brabandt,
            closes #9762)
diff --git a/nsis/gvim.nsi b/nsis/gvim.nsi
index 649c695..c32a886 100644
--- a/nsis/gvim.nsi
+++ b/nsis/gvim.nsi
@@ -348,6 +348,9 @@
 !if /FileExists "${VIMSRC}\vim${BIT}.dll"
 	File ${VIMSRC}\vim${BIT}.dll
 !endif
+!if /FileExists "${VIMRT}\libsodium.dll"
+	File ${VIMRT}\libsodium.dll
+!endif
 	File /oname=install.exe ${VIMSRC}\installw32.exe
 	File /oname=uninstall.exe ${VIMSRC}\uninstallw32.exe
 	File ${VIMSRC}\vimrun.exe