libmicrohttpd: fix build on riscv

Define _REENTRANT like already done for ARC otherwise pthread detection
will fail

Fixes:
 - https://autobuild.buildroot.org/results/910dc7164312637bf4dcd89e4cb476ae198466c8



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>