Fix compilation error when using #define TEST_NO_MAIN

[manugoyal]

I noticed the following compilation error when utilizing the TEST_NO_MAIN macro to split test definitions over multiple files:

../third_party/acutest/include/acutest.h:1991:5: error: extraneous closing brace ('}')
    }  /* extern "C" */
    ^
1 error generated.

To reproduce, create the following files in one directory.

tests.h

#pragma once

void test_failure();

tests.cc

#include "tests.h"

#define TEST_NO_MAIN
#include "acutest.h"

void test_failure() {
    TEST_CHECK(1 == 2);
}

test_runner.cc

#include "acutest.h"

#include "tests.h"

TEST_LIST = {
    { "test_failure", test_failure },
    { NULL, NULL },
};

Before this change, I see the error:

$ c++ -I ../third_party/acutest/include tests.cc test_runner.cc
In file included from tests.cc:4:
../third_party/acutest/include/acutest.h:1991:5: error: extraneous closing brace ('}')
    }  /* extern "C" */
    ^
1 error generated.

Afterwards, I get the desired output:

$ c++ -I ../third_party/acutest/include tests.cc test_runner.cc
$ ./a.out
Test test_failure...                            [ FAILED ]
  tests.cc:7: 1 == 2... failed
FAILED: 1 of 1 unit tests has failed.

[amgross]

Seems like my bug in #70