distro/rpm: add BuildRequires: gcc-c++

Reference: https://fedoraproject.org/wiki/Changes/Remove_GCC_from_BuildRoot
19 jobs for update-rpm-spec in 12 minutes and 11 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Build
passed #151527
amd64 linux docker
build:clang:linux:amd64

00:01:31

passed #151526
amd64 linux docker
build:linux:amd64

00:01:31

passed #151528
amd64 linux docker allowed to fail
srpm

00:00:51

 
  Test
passed #151545
amd64 linux docker
deckard:linux:amd64

00:06:02

passed #151542
privileged
distro:epel-7

00:02:30

passed #151541
privileged
distro:fedora-27

00:03:19

passed #151534
docker
doc

00:00:56

passed #151533
amd64 linux docker
installcheck:linux:amd64

00:01:43

passed #151536
amd64 linux docker
installcheck:valgrind:linux:amd64

00:02:04

passed #151530
docker
lint:c

00:02:52

failed #151531
docker allowed to fail
lint:clang-scan-build

00:01:37

passed #151529
docker
lint:lua

00:00:47

passed #151539
amd64 linux docker
respdiff:iter:tcp:linux:amd64

00:04:11

passed #151540
amd64 linux docker
respdiff:iter:tls:linux:amd64

00:04:05

passed #151538
amd64 linux docker
respdiff:iter:udp:linux:amd64

00:03:59

passed #151532
amd64 linux docker
test:linux:amd64

00:01:34

passed #151537
amd64 linux docker
test:linux:amd64:valgrind

00:01:37

failed #151535
amd64 linux docker
deckard:linux:amd64

00:01:30

 
  Coverage
passed #151543
amd64 linux docker
coverage

00:00:21

70.2%
 
Name Stage Failure
failed
lint:clang-scan-build Test
                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
5 warnings generated.
CCLD daemon/kresd
scan-build: 29 bugs found.
scan-build: Run 'scan-view /builds/knot/knot-resolver/scan-results/2018-07-16-093844-14-1' to examine bug reports.
Uploading artifacts...
scan-results: found 34 matching files
Uploading artifacts to coordinator... ok
id=151531 responseStatus=201 Created token=yQt84nQ2
ERROR: Job failed: exit code 1