acinclude.m4: Fix skb_get_hash function detection

acinclude.m4: Fix skb_get_hash function detection

Commit e2f3178f0582 ("datapath: Add support for kernel 3.14.") added
support for 3.14 kernels and a new OVS_GREP_IFELSE check for the
"skg_get_hash" function in the process. "skb_get_hash" was introduced
in the Linux kernel commit 3958afa1b272 ("net: Change skb_get_rxhash to
skb_get_hash") which exists in >=3.14 but the OVS_GREP_IFELSE macro
also matches the "skb_get_hash_raw" function which exists in older
kernels. As a result of which, the check makes the build system
behave as if the "skb_get_hash" function is available in these older
kernels leading to build failures. We fix this by explicitly checking
for "skb_get_hash(" which matches the function definition.

Signed-off-by: Markos Chandras <mchandras@suse.de>
Signed-off-by: Jesse Gross <jesse@kernel.org>