debian/rules: use multiline search to look for Build-Depends

When Build-Depends was split into multiple lines (in commit
8f5c665744e58d6d51a1e86de55c1399f51cc1c3), the grep for
libgoogle-perftools-dev broke.  Replace grep with perl for multiline
matching.

Fixes: #4818
Signed-off-by: Dan Mick <dan.mick@inktank.com>