nasm: fix build w/ gcc81

Fixes error: 'pure' attribute on function returning 'void'

Signed-off-by: Jürgen Buchmüller <pullmoll@t-online.de>

srcpkgs/nasm/patches/gcc81-pure_func.patch

@@ -0,0 +1,11 @@
+--- include/nasmlib.h	2017-11-29 20:44:08.000000000 +0100
++++ include/nasmlib.h	2018-05-22 11:26:47.909787252 +0200
+@@ -191,7 +191,7 @@
+  * seg_init: Initialise the segment-number allocator.
+  * seg_alloc: allocate a hitherto unused segment number.
+  */
+-void pure_func seg_init(void);
++void seg_init(void);
+ int32_t pure_func seg_alloc(void);
+ 
+ /*