cgen: fix #preinclude, add test case for #postinclude too

vlib/v/gen/c/cgen.v

@@ -570,6 +570,7 @@ pub fn gen(files []&ast.File, mut table ast.Table, pref_ &pref.Preferences) GenO
 	}
 	b.write_string2('\n// V comptime_definitions:\n', g.comptime_definitions.str())
 	b.write_string2('\n// V typedefs:\n', g.typedefs.str())
+	b.write_string2('\n // V preincludes:\n', g.preincludes.str())
 	b.write_string2('\n// V cheaders:', g.cheaders.str())
 	if g.pcs_declarations.len > 0 {
 		b.write_string2('\n// V profile counters:\n', g.pcs_declarations.str())

vlib/v/gen/c/testdata/postinclude_example.out

@@ -0,0 +1 @@
+Hi from V, p: 42

vlib/v/gen/c/testdata/postinclude_example.vv

@@ -0,0 +1,10 @@
+module no_main
+
+#postinclude "@VEXEROOT/vlib/v/gen/c/testdata/postinclude_header.h"
+
+@[markused]
+pub fn f(p int) int {
+	assert p == 42
+	C.printf(c'Hi from V, p: %d\n', p)
+	return 987
+}

vlib/v/gen/c/testdata/postinclude_header.h

@@ -0,0 +1,7 @@
+#include <assert.h>
+
+int main( void ) {
+   	int res = no_main__f(42);
+	assert(res == 987);
+    return 0;
+}