v.pkgconfig: fix building standalone pkgconfig (#14825)

* Update main.v

* add test to prevent future regressions

Co-authored-by: Delyan Angelov <delian66@gmail.com>

vlib/v/pkgconfig/bin/pkgconfig_test.v

@@ -0,0 +1,17 @@
+import os
+
+const vexe = os.getenv('VEXE')
+
+const vroot = os.dir(vexe)
+
+fn test_pkgconfig_can_be_compiled() ? {
+	tmp_exe := os.join_path(os.temp_dir(), '${os.getpid()}_pkgconfig.exe')
+	pkgconfig_v_file := os.real_path(os.join_path(vroot, 'vlib/v/pkgconfig/bin/pkgconfig.v'))
+	assert !os.exists(tmp_exe)
+	res := os.system('${os.quoted_path(vexe)} -o ${os.quoted_path(tmp_exe)} ${os.quoted_path(pkgconfig_v_file)}')
+	if res != 0 {
+		assert false
+	}
+	assert os.exists(tmp_exe)
+	os.rm(tmp_exe)?
+}

vlib/v/pkgconfig/main.v

@@ -3,7 +3,7 @@ module pkgconfig
 import flag
 import strings
 
-struct Main {
+pub struct Main {
 pub mut:
 	opt         &MainOptions
 	res         string