changelog.txt : add missing message

- gas64 : SGN type is INTEGER for whole numbers instead LONG

changelog.txt

@@ -22,6 +22,7 @@ Version 1.10.0
 - fbc: dos: set DXE_LD_LIBRARY_PATH environment variable when calling DXE3GEN
 - release: target DOS gcc 9.3.0 and bin utils 2.30 for next release
 - Unix GETMOUSE() under TERM=linux now requires libgpm 1.20.4 or higher (libgpm.so.2), instead of the older libgpm.so.1 versions. (for compatibility with modern Debian/Ubuntu versions, and probably other distros, since 2008)
+- gas64 : SGN type is INTEGER for whole numbers instead LONG
 
 [added]
 - gas/gas64: '.cif_sections' and '.cif_' directives for stack unwinding (adeyblue)