papi.h: Fix warnings for -Wstrict-prototypes

[anustuvicl]

Warnings seen while compiling with gcc 7.5.0

/home/anustuv/papidev/papi-AP2/papi/src/install/include/papi.h:1224:4: warning: function declaration isn't a prototype [-Wstrict-prototypes]
    int PAPI_rate_stop(); /**< stops a running event set of a rate function */
    ^~~
/home/anustuv/papidev/papi-AP2/papi/src/install/include/papi.h:1241:4: warning: function declaration isn't a prototype [-Wstrict-prototypes]
    int PAPI_hl_stop(); /**< stops a running high-level event set */
    ^~~

[gcongiu]

looks good. rebase and merge please