parser: Fix colno after multiline strings

Without this commit, meson thinks that the `var` token in the code below
starts at a different column number than it actually starts, because the
old author forgot to account for the length of the triple quotes.

```
'''
some multiline strings
abc''' + var
```

1 files changed, 1 insertions, 1 deletions

diff --git a/mesonbuild/mparser.py b/mesonbuild/mparser.py
index 30caf703a..a0a8d273e 100644
--- a/mesonbuild/mparser.py
+++ b/mesonbuild/mparser.py
@@ -201,7 +201,7 @@ class Lexer:
                         lines = value.split('\n')
                         if len(lines) > 1:
                             lineno += len(lines) - 1
-                            line_start = mo.end() - len(lines[-1])
+                            line_start = mo.end() - len(lines[-1]) - 3
                     elif tid == 'eol_cont':
                         lineno += 1
                         line_start = loc