patch 8.2.2772: problems when restoring 'runtimepath' from a session file

Problem:    Problems when restoring 'runtimepath' from a session file.
Solution:   Add the "skiprtp" item in 'sessionoptions'.
diff --git a/src/option.h b/src/option.h
index 0b1183f..30053cc 100644
--- a/src/option.h
+++ b/src/option.h
@@ -864,6 +864,7 @@
 # define SSOP_CURSOR		0x4000
 # define SSOP_TABPAGES		0x8000
 # define SSOP_TERMINAL		0x10000
+# define SSOP_SKIP_RTP		0x20000
 #endif
 EXTERN char_u	*p_sh;		// 'shell'
 EXTERN char_u	*p_shcf;	// 'shellcmdflag'