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MINOR: [R] Avoid {glue}'s whitespace trimming

For a while now glue has been removing the tabs before: (this might be from https://github.com/tidyverse/glue/commit/8369f9afe09cc52847624d2e55f0f72ce64ef25a which was to change [a different thing we noticed with whitespace stripping](https://github.com/tidyverse/glue/issues/247)).

Regardless, the fix is easy enough (don't include new lines around the line we are glueing)

Before the change, the format of the relevant lines of arrowExports.cpp are:
```
static const R_CallMethodDef CallEntries[] = {
{ "_arrow_available", (DL_FUNC)& _arrow_available, 0 },
{ "_dataset_available", (DL_FUNC)& _dataset_available, 0 },
{ "_engine_available", (DL_FUNC)& _engine_available, 0 },
{ "_parquet_available", (DL_FUNC)& _parquet_available, 0 },
{ "_s3_available", (DL_FUNC)& _s3_available, 0 },
{ "_json_available", (DL_FUNC)& _json_available, 0 },
{ "_arrow_test_SET_STRING_ELT", (DL_FUNC) &_arrow_test_SET_STRING_ELT, 1},
```

When they should be:
```
static const R_CallMethodDef CallEntries[] = {
	{ "_arrow_available", (DL_FUNC)& _arrow_available, 0 },
	{ "_dataset_available", (DL_FUNC)& _dataset_available, 0 },
	{ "_engine_available", (DL_FUNC)& _engine_available, 0 },
	{ "_parquet_available", (DL_FUNC)& _parquet_available, 0 },
	{ "_s3_available", (DL_FUNC)& _s3_available, 0 },
	{ "_json_available", (DL_FUNC)& _json_available, 0 },
	{ "_arrow_test_SET_STRING_ELT", (DL_FUNC) &_arrow_test_SET_STRING_ELT, 1},
```

This change restores the indentation we expect there (so the file won't change on rebuilds)

Closes #12770 from jonkeane/whitespace

Authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>