Add `prefix` in pkg-config file

[diamant3]

I'm trying to port flatbuffers library to a library compatible for PSP and we specifically use pkg-config to find and use the library but i noticed this error:

Variable 'prefix' not defined in '/usr/local/pspdev/psp/lib/pkgconfig/flatbuffers.pc'

EDIT:

~This PR hopefully solve this error.~ Tested, this is the fix, here's my output:

diamant3@MSI:~/flatbuffers/build$ psp-pkg-config --libs --cflags flatbuffers
-I/usr/local/pspdev/psp/include -L/usr/local/pspdev/psp/lib -lflatbuffers

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[diamant3]

Already signed the CLA. thanks.

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